"Nowhere did Luther's character shine forth more strikingly than in this controversy on the Lord's Supper. Never were more clearly displayed that firmness with which he clung to a conviction which he believed to be Christian, his faithfulness in seeking for no other foundation than Scripture, the sagacity of his defence, his animated eloquence, and often overwhelming powers of argumentation. But never also were more clearly shown the obstinacy with which he adhered to his own opinions, the little attention he paid to the reasons of his opponents, and the uncharitable haste with which he ascribed their errors to the wickedness of their hearts, or to the wiles of the devil. " One or other of us," said he to the Strasburg mediator, " must be ministers of Satan—the Swiss or ourselves." [source]
I pointed out earlier that there are really only three main writings from Luther directed toward Zwingli (I still haven't found any letters from Luther to Zwingli). That is, there are three specific documents in which Luther took the time to write directly and at length against Zwingli and the Swiss. There are though, a few lesser known documents as well. Here is an overview of Luther's writings against Zwingli, including some of these lesser known writings:
"In June, 1526, he published his Preface to the German edition of the Swabian Syngramma, in which he identified Karlstadt, Zwingli, and Oecolampadius as three heads of a new sect" (WA 19, 457 ff., St. L. 20, 577 ff. The Syngramma itself is printed in German translation in St. L. 20, 522 ff.). [LW 37 Introduction, electronic edition]. This is book is more directed towards the argumentation of Oecolampadius rather than Zwingli. LW 36 explains,
"Another minor encounter in 1526 was Luther’s Preface to the German translation of a Latin document composed by Brenz on behalf of a number of Swabian pastors to refute the position of Oecolampadius. The document is known as the Swabian Syngramma. It was published in January, 1526, and Luther was greatly pleased with it. The German translation by Agricola together with Luther’s preface appeared in June. The preface warned against the “new dreams about the sacrament” and again urged steadfastness in the words of Christ. A little later a second German translation of the Syngramma appeared, apparently with Luther’s encouragement, and it carried a second and more emphatic preface by Luther because by this time (September, 1526) the Swiss were claiming Luther’s assent to their views." [LW 36:331]
"His first independent treatise on the controversy, appearing in early autumn, 1526, The Sacrament of the Body and Blood of Christ—Against the Fanatics, was simply a pastoral presentation of his views, edited by friends from three Easter sermons" [LW 37 Introduction, electronic edition].
"It is uncertain what part Luther himself had in the publication of this 'sermon.' Two copies of the three sermons he preached at Easter, 1526, are extant; apparently they were written down by his hearers. A comparison of these two copies with the text of the “sermon” published six months later indicates that a number of additions were made in preparing the materials for publication. These additions do not in all cases fit into the logic of the sermons themselves. This lack of continuity suggests that it was not Luther himself who prepared the sermons for publication. Certainly Luther could not have regarded them as a systematic polemic against the Swiss theologians. They were clearly intended as popular instruction for the laity and were adapted to the educational and doctrinal background of his Wittenberg congregation. It is therefore unlikely that Luther himself would have added to the title the phrase, 'Against the Fanatics.' Nor would he have chosen a title that covers only the first two of the sermons unless he had intended to omit the sermon about the confession of sins" [LW 36:332].
The complete sermon "Sacrament of the Body and Blood of Christ against the Fanatics" is available in LW 36.
[March 1527] Available in LW 37. This treatise can be found under different titles, more-a-less saying the same thing in English. For instance, Preserved Smith refers to it as "That these Words of Christ, 'This is my Body,' still stand against the Ranting Spirits" [The Life and Letters of Martin Luther, p.241]. In Robert Kolb's book Martin Luther as Prophet, Teacher, and Hero, Kolb documents how regularly this treatise appeared in versions of Luther's collected writings, and also makes note of it being republished, sometimes only partially, sometimes edited for polemical purposes, manipulating Luther's words (for instance, The Wittenberg Edition deleted Luther's criticism of Bucer from this treatise [see Kolb, 146].
"The text of the treatise, both Luther’s manuscript and the printed edition with annotations, is found in WA 23, 64–283; in modernized German in Br A 4, 335–480, and in St. L. 20, 762–893 (printed version alone). [WA D. Martin Luthers Werke. Kritische Gesamtausgabe (Weimar, 1883– ).Br A Luthers Werke für das christliche Haus, edited by Georg Buchwald et al., 4th edition (Leipzig, 1924). St. L. D. Martin Luthers sämmtliche Schriften, edited by Johann Georg Walch. Edited and published in modern German, 23 vols. in 25 (2nd ed., St. Louis, 1880–1910]" [LW 37:6].
 Available in LW 37. "This treatise was indeed his last published word on the controversy, until under provocation from Schwenkfeld he took up his pen once more in 1544, to indicate in his Brief Confession on the Holy Sacrament that he still maintained the same views. He paid no attention to the lengthy and bitter rebuttals by Oecolampadius and Zwingli, which, hastily written for the autumn fair and printed together in one volume, added nothing essentially new to the controversy: Concerning Dr. Martin Luther’s Book Entitled “Confession”: Two Answers, by John Oecolarnpadius and Ulrich Zwingli. Nor did he take notice of the host of lesser treatises that continued to appear against him"[LW 37: 157].
6. The Marburg Colloquy
The Landgrave's Chancellor, Johu Feige, opencd the Disputation, in a speech, in which he exhorted the members that they should act, as had been done on like occasions, when learned men came together, who had previously written somewhat sharply against each other, that is, they should banish from their minds all ill-humour and bitterness of feeling.
"Whoever should do this would, at the same time, discharge his duty, and obtain glory and commendation. Others, however, who disregarded unity, and who obstinately persisted in some notion once adopted by them (the mother of all heresies), would thereby afford indubitable evidence against themselves that the Holy Spirit did not rule in their hearts."
The Landgrave, so simply attired that no one could have taken him for a prince, took his seat at the same table, at which Zwingli and OEcolampad, on the one side, Luther and Melanchthon on the other, sat to decide whether the Reformed Evangelical Church, resting on one basis of faith, was henceforth to remain united, or whether it was to be rent into two great parties. The poet Cordus, cried in name of the Church to its here assembled leaders: "Puissant princes of the Word, whom the august hero Philip has called to avert from us schism, and to shew us the way of truth; the imploring church falls at your feet, drowned in tears, and conjures you, in Christ's name, to set forward the good cause, that the world may recognise in your resolutions the work of the Holy Ghost himself."
Before the Conference began, Luther took up a piece of chalk, and, in large letters, he wrote upon the table the words, " This is my body," with the object, doubtless, that, when arguments failed, he might all the more firmly cling to the outward letter, since, verily, he was resolved not to yield a hairbreadth." The Conference began between Luther and OEcolampad, Luther defending himself, in a long speech, against the imputation that he, in any respect, agreed with the doctrine of the Supper held by his opponents; he was at variance with them here, and would be for ever so, Christ himself having said, with sufficient clearness, "Take, eat, this is my body." By the letter of these words he would abide. If his adversaries had anything to advocate against the truth he would hear it, and answer it.
OEcolampad replied, after calling upon God for illumination, "It is undeniable that, in the Word of God, figurative modes of expression occur; thus, for example, " John is Elias," " The rock was Christ," " I am the vine." A similar figure is contained in the words, " This is my body."
Luther grants there are tropes in the Bible, but the latter passage is not one of these. He inquires: Why should the spiritual partaking exclude the corporeal?
OEcolampad : Christ teaches, the Jews, John vi., who thought He exacted from them the eating of His real body, and the drinking of His real blood, that He was, in verity, eaten and drunk when He was believed upon, for that His flesh profited nothing ! Now, that which Christ rejected in John vi. He cannot well be supposed to have admitted, or commanded, in the words of the Holy Supper.
Luther: The Jews thought they were to eat Christ like a piece of "roasted pork." By the spiritual partaking, the corporeal is not annulled.
OEcolampad: To impute such a sense to the words of Scripture is to give them a sense somewhat gross. That Christ is in the bread is a notion, and no subject of faith; it is dangerous to ascribe so much to the outward thing.
Luther: If we, at God's command, raise a straw-halm, or a horse-shoe, from the ground, it is a spiritual act. We must regard Him who speaks, not that which is said. God speaks, and miserable man must listen. God commands, the world has to obey, and we all ought to kiss the Word, and not take upon ourselves to look for arguments.
OEcolampad: But of what use is the partaking by the mouth when we have that by the Spirit ?
Luther: I do not concern myself as to what we require, I look only at the words as they stand written: " This is my body." It is to be believed and done unconditionally. It must be done. If God were to command me to eat dung, I should do it, knowing well that it would be wholesome for me.
Zwingli now took part in the dialogue. He began by administering a sharp rebuke to Luther for his declaration at the very outset of the debate, that he was resolved not to depart from the opinion he had formed; for, in this manner, all farther instruction out of the Scriptures was rendered impossible. Scripture must always be interpreted by Scripture. Were we to adhere to the letter of the text we must conclude that Christ had full brothers. The sentences of Holy Scripture are not dark or enigmatical, like (he oracular responses of the demons, but they are clear and plain, if we only compare the one with the other. He then went into a more minute exposition of the section in John vi., and drew from it the conclusion: " If the Lord here expressly testifies that His flesh profiteth nothing in the corporeal partaking of it, He certainly would neither have enjoined upon His disciples, nor upon us, in the Supper, the doing of a profitless thing, that is, the corporeal eating of His body. To this He says : ' When ye shall see the Sou of man ascend to where he was before,' from which they might conclude that they are not to eat really, or corporeally, of His flesh."
Luther: In the gospel, "brother" signifies a cousin, or a relation. The words of institution cannot be so explained. Christ says, " This is my body," and it must be so. When Christ says, " the flesh profiteth nothing," He is not speaking of His own but of our flesh.
Zwingli: The soul is nourished by the Spirit, not by the flesh.
Luther: The body is eaten by the mouth, the soul does not partake of it corporeally.
Zwingli: It is then a food of the body and not of the soul.
Luther: I have said, and say it again, the body is not corporeally eaten into our body, and will reserve it, whether the soul also eats it.
Zwingli: You say this, however, without being able to prove it by Scripture. Besides, you first denied that the soul eats the body, and now you will have it reserved.
Luther: Your whole object is to catch me in my words.
Zwingli: No; but you speak of things that contradict each other, and it is necessary to point out the truth.
Luther: I abide by the words of Christ, "This is my body." They are the words of God. If the Lord were to set before me wooden apples, and command me to eat them, I should eat them, knowing they would be wholesome for me, and I dare not ask ; why?
Zwingli now proved, by various passages of Scripture, that the sign is often put for the thing signified, and that the words of the Sacrament especially are to be so explained. He censured Luther for employing so silly an example as that of the wooden apples. Such illustrations were not in place. We know that God neither commands us to eat wooden apples nor dung as His body. The Word of God reveals to us His holy will; it is light, not darkness. God sets before us nothing incomprehensible, if if we will but only rightly understand His Word. Hence, if one passage is not clear to us, we must compare it with others, and, in this manner, investigate into the sense. Thus the Virgin Mary asked, Luke i. 34, "How shall this be?" and the angel answered her question. In the same manner the disciples asked, John vi . 52, "How can this man give us His flesh to eat?" Why should not we also endeavour to discover, from Scripture, how the words of the Holy Supper are to be understood? They have, however, been interpreted by Christ himself, who shewed in what manner His flesh was to be eaten, and His blood drunk.
Luther: We are not to examine whether is may be taken for signifies, for so we fall into interpretising; but we are to take the words in their simple sense, " This is my body." From thence, pointing at the words written before him, the devil himself cannot pull me. When I enter into subtle inquiries about their meaning, I lose my faith and become a fool. Wherefore, give glory to God, and take and believe the simple plain letters as they stand.
Zwingli: I exhort you likewise to give God the glory, by departing from the false interpretation you have put upon the words of Scripture, by an assumption of the very thing to be proven, petitio principii. Where is your major proposition, (that the words bear this sense,) proved ? We shall not so readily let the passage in the sixth chapter of John slip out of our hands, as it throws a steady light upon the point in dispute, and shows us distinctly how in truth and verity we are to eat Christ's flesh and drink His blood. Come, doctor, you must sing us another song than this, for this won't do.
Luther: You are becoming personal.
Zwingli: I ask you, Doctor, if Christ did not mean here to correct the misunderstanding of the Jews, who fancied they were to eat His real flesh and drink His real blood ?
Luther: Mr Zwingli, you mean to take me by surprise; the passage has nothing to do here.
Zwingli: Certainly the passage has to do here, and breaks your neck, Doctor.
Luther: Not so boastful, remember you are not in Switzerland now, but in Hesse, where necks are not so easily broken.
Zwingli: In Switzerland there is law and justice, as well as elsewhere, and no man's neck is broken there for naught, I have only made use of a common phrase, when I employed this expression to the effect that your case was gone, that you could do nothing but submit, seeing that the words of Christ in the sixth chapter of John totally overthrow your doctrine.
The Landgrave here interfered, saying to Luther, " I hope my learned friend the Doctor will not take ill what has been said." If Luther had but reflected on his usual threat, " we shall bring the villain to the gallows," he would have perceived that he had no great reason to complain of Zwingli's expression.
It being now exactly noon, the Conference adjourned till after dinner. In the afternoon, Zwingli read the following extract from Luther's Sermon on the sixth chapter of the Gospel of John, " Christ himself says the flesh profiteth nothing, and again, my flesh gives life, how do we reconcile this? The Spirit reconciles it. Christ means that the corporeal eating of His flesh profits nothing, nothing profits but the faith that He has given His flesh for me, and shed his blood for me. If I believe that Christ is the true Son of God, that He hath descended from heaven, shed His blood for me, saved me, made me righteous and alive from the dead, I have enough." Melanchthon had explained this passage in a similar way.
Luther: I make no inquiry how Melanchthon and myself formerly explained this passage. Prove to me that when Christ says " This is my body," it is not his body. I take my stand and abide, and not without grounds, by the words " this is my body," but yet I do not the less acknowledge that Christ's body is in heaven, and also in the Sacrament. I am not concerned as to its being against reason and against nature, if it be not against faith.
Zwingli: This statement, however, undoubtedly contravenes the articles of faith, " He hath ascended to heaven," &c. &c. If Christ's body be in heaven, how can it at the same time be in the bread? God's word teaches us that Christ was in all points made like unto His brethren, Heb. ii. 17. His body then cannot at the same time be in different places, because this is contrary to the nature of a real body.
Luther: If He hath been in all respects like to us, then He has had a wife and black eyes. I have said it before, and say it again : I will have nothing to do with the Mathematica!
Zwingli: I am not speaking of the Mathematica, but of the Word of God. He then, in order to show that Christ, although of Divine nature, had taken upon Him the form of a servant, and been made like to us, cited in the Greek text the passage from Philip, ii. 7.
Luther: Let Greek alone, quote it in Latin or in German.
Zwingli: Excuse me; during the last twelve years, I have only made use of the Greek New Testament, If Christ then has been made like to us, this is to be understood of His human nature. Accordingly, His body, like every other human body, is finite.
Luther: I admit that Christ's body is finite.
Zwingli: If it is finite, it is also limited, and can only be at one and the same time in one place, that is in heaven, and not in the bread. But now you teach that the body of Christ is everywhere present,
Luther: You always seek to entrap me. If I speak of the body of Christ, I will not have it that one speak or think of a place I will not have it at all.
Zwingli: What sort of language is this? Are we only to have what you will, Doctor ?
Luther: The schoolmen have also maintained that a finite body can be in several places at once. The universe is a body, aud yet it cannot be said that it is in any definite place.
Zwingli: It ill becomes you, Doctor, to have recourse to the onions and flesh-pots of Egypt, to the Sophists ; I, for my part, pay no regard to the Sophists. If you say that the universe is nowhere, I beg all intelligent men to test the truth of this assertion ; you were, however, to make good that the body of Christ was at one and the same time in more than one place.
Luther: Christ says, "this is my body." Now the Sacrament is dispensed in many places at once, in which one partakes not only of bread, but of the true body of Christ, hence Christ's body is in many places at once.
Zwingli: This does not follow from the words of Christ, the sense of which we are here investigating. You ever assume that your understanding of these words, which we declare to be an erroneous and false one, is the right and infallible one, and proceeding from this false assumption, you avail yourself of the sophism of reasoning in a circle. Instead of which, your proper business is to prove and establish your understanding of these words to be the true and right one. That the body of Christ, however, is limited or circumscribed like our own, and consequently, can only be at one time in one place, is a doctrine taught us by the Fathers. Thus Fulgentius says: " The Son of (Joel has taken upon himself the quality of real humanity, and yet not less that of real divinity. Born of His mother in time He is yet from all eternity, in virtue of the Godhead which He bus from the Father. Born of man, He is man, and bound to a definite place ; as He emanates from the Father, He is God, and consequently omnipresent . In His human nature, He was when on earth absent from heaven, and He left the earth when He ascended to heaven : in His divine nature, He abode in heaven when He descended, nor did Heleavethe earth when He ascended." You, however, dear Doctor, have written ere now, " Every thing is full of the body of Christ," and "if Christ had not suffered in His divine nature, He were not my Redeemer."
Luther: Fulguntius is not here speaking of the Supper. Moreover he calls the Supper a sacrifice too, and yet it is none.
Zwingli: Fulgentius is here speaking of the qualities of Christ's humanity, and maintains that it necessarily follows that as man He can only be corporeally present in one place. If that is true in respect of Christ's humanity in general, it is likewise true of His presence in the Supper. When Fulgentius, however, terms the Supper a sacrifice, he does it in the same sense as Augustine, who calls it a sacrifice as he himself explains his meaning, because it is a commemoration of the once offered sacrifice of Christ.
Luther, after a few struggles, was obliged to admit this, but fell immediately into his old habit of reasoning in the circle, and drew the conclusion that Christ's body may be in many places at once, because He says, " this is my body," consequently He is now there in the bread.
Zwingli quickly rejoining: Is He there in the bread ? then there is surely in one place. Methinks, Doctor, I have you.
Luther: As God will, let Him be in one place or not, I leave that with God; to me it is enough, and I abide by it, that He says, " This is my body."
Zwingli: It is evident to every one that you argue from a false assumption, that you describe a reasoning in a circle, and that you thus, intrenched in your own opinion, obstinately close your eyes against all instruction from the Word of God. This is but a miserable spite on your part, Doctor. In like manner might some wilful disputant misinterpret the words of our Lord to his mother, " Woman, behold thy son," persist in repeating them, and, despite all remonstrances, never cease crying, No, no, you must take the words of Christ as they stand, and hold simply by them, " Woman, behold thy son." Would he achieve aught else here, but a miserable perversion of the words of Christ ? It is just what you are doing, Doctor. The holy Augustine writes : " We dare not believe that Christ in human form is everywhere present, we dare not, to establish His divinity, abstract the reality from His body. Christ as God is omnipresent, yet by reason of His true body He is in one place, in heaven."
Luther: Augustine is not here speaking of the Sacrament. The hotly of Christ in the Supper is not as in one place.
Zwingli declining to reason any farther with an opponent who withdrew himself from every species of close and consecutive argument, and who overleapt with such wonderful audacity the manifold contradictions into which he plunged, OEcolampad now took it upon him to answer Luther. In reply to Luther's last assertion, which had been already thoroughly disproved by Zwingli, and which was in direct contradiction to his own former admissions, OEcolampad observed : "If the body of Christ is not locally in the Supper, then it is not there as a real body, for, as is well known, it belongs to the essence of a body to be in one definite place. Let us examine, in all friendship, what kind of presence this is of the body of Christ."
Luther: You will not bring me a single step farther. If you have Fulgentius and Augustine on your side, we have the rest of the Fathers.
OEcolampad: Please name these Fathers, and quote the passages you refer to. We trust we shall be able to prove to you that they are of our opinion.
Luther: We decline naming them. Augustine wrote the passage you have quoted in his youth, it is moreover very unintelligible. Besides, I do not concern myself as to what the Fathers teach on this head, but I abide by the words of Christ, (Here he pointed again to the words written in chalk upon the table, " This is my body.") See, so they run. You have not driven us . out of this stronghold, as you proudly imagined you would do, and we concern ourselves no farther about proofs.
OEcolampad: If it be thus, the Conference had better be closed. We have appealed to the Fathers of the Church for the purpose of shewing that we have advanced no new doctrine. We do not build upon them, but upon the Word of God. Every one knows who Augustine was, and that when he expressed his sentiments, he not only delivered his own opinions, but those of the whole Church of his day.
Thus the Conference concluded. The Chancellor, Feige, who for his part adhered to the Zwinglian doctrine, was dismayed at the upshot. But, even yet, he exhorted both parties earnestly to cultivate peace, as he had done at the beginning, and entreated them to think of measures which might promote unity. Luther observed, " I know of no other measures but that they give glory to the Word of God, and believe what we believe."
The Swiss replied: " We cannot do so, for our conscience forbids it . We believe that Christ's body and blood are present in the Supper to the believing soul, but not in the bread and wine." Luther: "We have then done with you, and commend you to the just judgment of God: He will discover who is right."
Then OEcolampad said: "We do the same, and have done with you."
Zwingli, however, was so deeply moved by Luther's obduracy of temper, that he was unable to articulate a single word; his eyes, as every one saw, were swimming in tears.