I haven't come across a lot of material on whether or not Luther believed a Christian can lose salvation. Yes, of course, those who spend their days attacking "OSAS" are prone to mention Luther (ironically, most of the time despising Luther for other reasons). It's not surprising to read quotes from him that appear to advocate a "perseverance of the saints" as well as quotes suggesting loss of salvation. Luther's use of paradox allows for such differing statements. I am aware that current Lutherans do say a Christian can "fall from faith." Also, I'm aware of statements like these:
“Although Luther agreed that the merits of Christ were the sole basis of a man’s justification, and that it did not depend in any way on a man’s deeds, Luther still thought that a man could lose his justification if he totally and finally turned away from Christ. Since God’s gift of forgiveness of sins and eternal life was appropriated by faith, if a man decided not to rest his eternal destiny in Christ, and totally turned against Him, Luther believed that only then would a man lose his salvation. In other words, the only sin that Luther thought would cause a man to lose his salvation was the sin of unrepentant apostasy” (Catholics and Protestants: Do They Now Agree?, John Ankerberg and John Weldon).
Without ever really delving into it, I've followed the pack so to speak, and have agreed that Luther held a Christian can actually lose salvation due to disbelief or a reliance on works righteousness. On the other hand, it really isn't as simple as some make it out to be. Many Lutherans are rightly agitated when the Reformed try to present Luther as a 5 point Calvinist. However, I as a Reformed person tend to be agitated by those who ignore evidence, or don't ask interpretive questions about contexts. I'm particularly not at all fond of attempts to wiggle out of Luther's strong statements in The Bondage of the Will regarding predestination. I've had Luther supporters tell me certain English translations aren't accurate, or that Luther went too far, and later in his life he took a different position. I'm well aware of Luther's many exhortations not to probe into the secret council of the hidden God, but as to doctored English translations and Luther changing his mind, I've not seen convincing proof.
Luther Believes in Losing Salvation
Here are a few statements I've come across. This website, dedicated to advocating the possibility of a believer's loss of salvation, uses the following statements from Luther (emphasis theirs):
Even Martin Luther, who is claimed by Calvinists as one of their own, acknowledged the possibility of a Christian falling away into unbelief. Here are a few quotes, beginning with Luther's comment on the statement of the Lord's prayer, "lead us not into temptation."
"We have now heard enough what toil and labor is required to retain all that for which we pray, and to persevere therein, which, however, is not achieved without infirmities and stumbling. Besides, although we have received forgiveness and a good conscience and are entirely acquitted, yet is our life of such a nature that one stands to-day and to-morrow falls. Therefore, even though we be godly now and stand before God with a good conscience, we must pray again that He would not suffer us to relapse and yield to trials and temptations. ... Then comes the devil, inciting and provoking in all directions, but especially agitating matters that concern the conscience and spiritual affairs, namely, to induce us to despise and disregard both the Word and works of God to tear us away from faith, hope, and love and bring us into misbelief, false security, and obduracy, or, on the other hand, to despair, denial of God, blasphemy, and innumerable other shocking things. These are indeed snares and nets, yea, real fiery darts which are shot most venomously into the heart, not by flesh and blood, but by the devil. Great and grievous, indeed, are these dangers and temptations which every Christian must bear, even though each one were alone by himself, so that every hour that we are in this vile life where we are attacked on all sides, chased and hunted down, we are moved to cry out and to pray that God would not suffer us to become weary and faint and to relapse into sin, shame, and unbelief. For otherwise it is impossible to overcome even the least temptation. This, then, is leading us not into temptation, to wit, when He gives us power and strength to resist, the temptation, however, not being taken away or removed. For while we live in the flesh and have the devil about us, no one can escape temptation and allurements; and it cannot be otherwise than that we must endure trials, yea, be engulfed in them; but we pray for this, that we may not fall and be drowned in them." (Martin Luther, Large Catechism XII, On the Lord's Prayer, 6th Petition).
"Through baptism these people threw out unbelief, had their unclean way of life washed away, and entered into a pure life of faith and love. Now they fall away into unbelief" (Martin Luther, Commentary on 2 Peter 2:22).
"Verse 4, "Ye are fallen from grace." That means you are no longer in the kingdom or condition of grace. When a person on board ship falls into the sea and is drowned it makes no difference from which end or side of the ship he falls into the water. Those who fall from grace perish no matter how they go about it. ... The words, "Ye are fallen from grace," must not be taken lightly. They are important. To fall from grace means to lose the atonement, the forgiveness of sins, the righteousness, liberty, and life which Jesus has merited for us by His death and resurrection. To lose the grace of God means to gain the wrath and judgment of God, death, the bondage of the devil, and everlasting condemnation." (Martin Luther, Commentary on Galatians, 5:4).
As to the first quote from Luther's Large Catechism, I don't see anything particularly against a saint persevering. In fact, if you note the words they emphasised (in black), the point being made is the Devil is he who seeks to "tear us away from faith, hope, and love and bring us into misbelief" and to the "denial of God [and] blasphemy."
As to the second quote from Luther's Commentary on 2 Peter 2:22, Luther is expounding on false teachers, and these he takes to be initially those from "the schools of higher learning." He describes them as having a false faith. He also will apply these verses to the papacy, monks, and priesthood. In commenting on "even denying the master who bough them," he states:
"Behold, what powerful words St. Peter uses! He says: “They deny the Master who bought them.” They should be under Him as under a Master who owns them. But now, even though they believe that He is a Lord who has ransomed all the world with His blood, yet they do not believe that they are ransomed and that He is their Master. They say that although He ransomed and redeemed them, this is not enough; one must first make amends and render satisfaction for sin with works. Then we say: “If you take away your sin yourself and wipe it out, what, then has Christ done? You surely cannot make two Christs who take away sin. He should, and wants to, be the only One who puts sin aside. If this is true, I cannot make bold to wipe out sin myself. But if I do this, I cannot say or believe that Christ takes it away.” This amounts to a denial of Christ. For even if they regard Christ as a Lord, yet they deny that He redeemed them" [LW 30:171].
The quote being utilized is tied together with Luther's understanding of baptism (which I've outlined here). It doesn't have anything to do with a Christian walking in true faith for countless years, and then losing that faith.
As to the third quote from Luther's Commentary on Galatians, this is the only bone given that has some meat on it. If one reads through the entire passage, it indeed does appear Luther's is saying that if one falls from grace, one loses salvation. Luther goes on to say:
These words, “You have fallen away from grace,” should not be looked at in a cool and careless way; for they are very emphatic. Whoever falls away from grace simply loses the propitiation, forgiveness of sins, righteousness, freedom, life, etc., which Christ earned for us by His death and resurrection; and in place of these he acquires the wrath and judgment of God, sin, death, slavery to the devil, and eternal damnation. This passage is a powerful support and reinforcement for our doctrine of faith or the doctrine of justification; and it gives us marvelous comfort against the ragings of the papists, who persecute and condemn us as heretics because we teach this doctrine. This passage really ought to strike terror into all the enemies of faith and grace, that is, all the partisans of works, to make them stop persecuting and blaspheming the Word of grace, life, and eternal salvation. But they are so calloused and obstinate that “seeing they do not see, and hearing”—this horrible sentence pronounced against them by the apostle—“they do not hear” (Matt. 13:13). Therefore let us let them alone, for they are blind leaders of the blind (Matt. 15:14) [LW 27:19].
Here's one other Luther quote I found being put forth on a Reformed web page, one wonders if a believer could actually chose unbelief (is Luther speaking rhetorically?):
Even if he would, he could not lose his salvation, however much he sinned, unless he refused to believe. For no sin can condemn him save unbelief alone. All other sins, so long as the faith in God’s promise made in baptism returns or remains, are immediately blotted out through that same faith, or rather through the truth of God, because he cannot deny himself if you confess him and faithfully cling to him in his promise. But as for contrition, confession of sins, and satisfaction, along with all those carefully devised exercises of men: if you rely on them and neglect this truth of God, they will suddenly fail you and leave you more wretched than before. For whatever is clone without faith in God’s truth is vanity of vanities and vexation of spirit [Eccles. 1:2, 14]" [LW 36: 60].
An interesting analysis of this quote can be found here.
The quote though that settles the matter is article 40-43 of the Smalcald articles:
40 In the case of a Christian such repentance continues until death, for all through life it contends with the sins that remain in the flesh. As St. Paul testifies in Rom. 7:23, he wars with the law in his members, and he does this not with his own powers but with the gift of the Holy Spirit which follows the forgiveness of sins. This gift daily cleanses and expels the sins that remain and enables man to become truly pure and holy.
41 This is something about which the pope, the theologians, the jurists, and all men understand nothing. It is a teaching from heaven, revealed in the Gospel, and yet it is called a heresy by godless saints.
42 Some fanatics may appear (and perhaps they are already present, such as I saw with my own eyes at the time of the uprising)1 who hold that once they have received the Spirit or the forgiveness of sins, or once they have become believers, they will persevere in faith even if they sin afterwards, and such sin will not harm them. They cry out, “Do what you will, it matters not as long as you believe, for faith blots out all sins,” etc. They add that if anyone sins after he has received faith and the Spirit, he never really had the Spirit and faith. I have encountered many foolish people like this and I fear that such a devil still dwells in some of them.
43 It is therefore necessary to know and to teach that when holy people, aside from the fact that they still possess and feel original sin and daily repent and strive against it, fall into open sin (as David fell into adultery, murder, and blasphemy), faith and the Spirit have departed from them.
44 This is so because the Holy Spirit does not permit sin to rule and gain the upper hand in such a way that sin is committed, but the Holy Spirit represses and restrains it so that it does not do what it wishes. If the sin does what it wishes, the Holy Spirit and faith are not present, 45 for St. John says, “No one born of God commits sin; he cannot sin.” Yet it is also true, as the same St. John writes, “If we say we have no sin, we deceive ourselves, and the truth is not in us.”
Luther Believes The Saints Will Persevere
Now, on the other hand, I recently read through Luther's sermons on John 17. Here are a few interesting quotes suggesting a perseverance of the faith in the heart of a true believer.
But to this He adds "to those whom You gave Me from the world." For just as no one reveals this and causes it to be preached except Him, so no one is able to understand or accept this revelation except those who have been given to Him. The rest despise it or take offense, persecute and blaspheme it. All this is now said for our sakes, who have the Word of Christ and cling to it. And it is an excellent, comforting text for all timid, fearful consciences,especially for those who are troubled and afflicted with high temptations concerning their predestination.
If anyone wants to know whether he is elect or how he stands with God, let him simply look to the mouth of Christ, namely, to this passage and ones like it. For though one cannot say with certainty who will be [called] in the future or who will finally endure, it is nonetheless certain that those who have been called and have come to hear this revelation (that is, Christ's Word), as long as they also accept it seriously (that is, they regard and believe it as entirely true). They are the ones given to Christ by the Father. Those who are given to Him He will uphold and protect so that they will not perish, as He says in John 6 [:39]: "This is the will of the Father, who sent Me, that I should lose nothing of all that He has 'given Me." And later in this chapter [John 17:12]: "Those whom You have given Me I have guarded, and not one of them has been lost except the son of perdition" Again, in John 10 [:28], He speaks of the sheep who hear His voice: "I give them eternal life. and they shall never perish, and no one shall tear them out of My hand."
For you must assuredly believe that there is no higher grace and divine work than that someone comes to hear the Word of Christ gladly with all his heart and takes it seriously, regarding it as great and precious. For, as has been said. not everyone concerns himself with this, nor does it come from human understanding or choice. It takes more than reason and free will to be able to grasp and accept it, as Christ says in John 6 [:44]: "No one can come to Me unless the Father draws him" And again [John 6:45]: "Whoever hears it and learns it from My Father comes to Me." These words, even though they sound harsh toward false Christians, are, nevertheless, sweet and comforting to upright hearts that hold His Word dear, if one looks into Christ's heart and mind from which they flow. For He wants to indicate, as has been said, that it is not man's will and intention that make one cling to Christ and become His disciple, but it is God's work and power.
This is readily proved by looking out into the wide world and seeing how few there are who value Christ's Word and hold it dear, particularly where might, wisdom, holiness, etc., rule. There is nothing more despised or accursed on the face of the earth than the dear Gospel. The world in its wisdom is able to censure it so masterfully, to mock and scorn it so disdainfully, to libel and slander it so venomously and sharply, to persecute it so fiercely and bitterly, that, in sum, no folly, no vice, no aberration, no devil is so hated as Christ is. Man is able to tolerate, ignore, excuse, and prettify all sorts of sects, blasphemy of God, public shame, and vice. But Christ must take all this upon Himself and bear it; on Him all people pour out their venomous, insatiable hatred and resentment. Therefore, do not take it as a small comfort but as a sure and certain one that if you feel that you love Christ and His Word and with all your heart desire to abide steadfast in it, you are among the little flock that belongs to Christ and shall not be lost.
Now if you are also tempted by such thoughts as, "Yes, even though I hold Christ dear and gladly hear Him, who knows whether I am reconciled with the Father in heaven?"—this, too. He will clear away, saying: "You fool, you would be entirely unable to delight in My Word or revelation if this had not been given you by the Father! Don't you hear that it is His own work and grace? For He has already taken you out of the world and given you to Me; that is. He has put it into your heart to hear Me gladly and hold My Word in love and esteem. There you have everything. What more is there to look for? Only take heed lest you fall away." In sum, whoever is clinging to Christ possesses sheer grace and cannot be lost, even if out of weakness he should fall like St. Peter, so long as he does not despise the Word like the crude spirits who boast of the Gospel yet pay no attention to it. For no one may apply this comfort to himself except poor, distressed, tempted hearts that desire to be reconciled with God, and hold Christ dear, and do not willfully set themselves against His Word but are sorry that it is blasphemed or
persecuted. [LW 69:50-51]
I am praying for them, and I do not pray for the world (John 17:9)
From this let us also take comfort, be joyful and of good cheer, and in firm faith conclude that those for whom Christ is praying will certainly be delivered and preserved against the devil's fury and rage, as well as against sin and every temptation. [LW 69: 61-62]
The ones You have given me (John 17:11)
Thereby we know that God himself has led us to hear Christ, and our salvation does not depend on ourselves but is in God's hand, "from which no one can snatch them [John 10:29]. Therefore he means: "Since You gave them to Me that they might become my disciples and have called them to true holiness, I pray that You will henceforth preserve them in it, that they may not become unsanctified or polluted and be misled in any error" [LW 69:75].
John 6: 38. For I have come down from heaven, not to do My own will, but the will of Him who sent Me;39. and this is the will of Him who sent Me, that I should lose nothing of all that He has given Me, but raise it up at the Last Day.
The fatherly will of which Christ speaks here includes and teaches that He, the Lord Christ, will not lose any of those who come to Him and are given to Him, that is, those who believe in Him, but that all of them will be saved and live eternally. Thus Christ says at another place: “For this is the will of My Father, that everyone who sees the Son and believes in Him should have eternal life; and I will raise him up at the Last Day” (John 6:40). This surely does not mean to be cast out, but to be kept with Him. This is a far different will from what the Law demands of us. It is necessary, therefore, to distinguish between the wills of God. The will of God reflected in the text is this, that no believer in Christ is to be lost. It pictures God as kindly disposed to us and banishes all lightning, thunder, hail, yes, all wrath and disfavor of God. It reveals God’s gracious resolve that all who see the Son and believe in Him shall be preserved, saved, and well supplied. God does not deal with them according to justice and its just verdict and punishment, but He entertains a gracious will toward them. God does not come to punish, but His will in Christ is only the gracious will of the Father, which kindly invites us to come to Him. [LW 23:63].
It does indeed appear Luther believed in the loss of salvation. Even with those strong comments from LW 69, he includes statements like "Only take heed lest you fall away" and "so long as he does not despise the Word."
On the other hand, My understanding is that Luther did indeed attribute double predestination to the "hidden God", so in some sense for Luther, there are a specific determined number of people God chooses to save, that will be saved, and it can not be otherwise, and it has nothing to do with their "free" choices:
“For Luther the assertion that God is God implicitly includes the fact that God alone works all in all together with the accompanying foreknowledge…. This determines not only man's outward but also his inner fate, his relationship to God in faith or unfaith, in obedience or disobedience. Here too man is completely in God's hands. Luther finds the biblical basis for this particularly in I Corinthians 12:6, "God works all in all." Luther expands the sense of this passage far beyond Paul's meaning in its original setting. It appears very frequently in Luther's thought.
The Bible in addition bears witness, and experience confirms the fact, that men actually relate themselves differently to the word of God. Some are open to faith; others remain closed to it. Accordingly, the Bible expects human history to end in a twofold way. Not all will be blessed; and many will be lost. Luther can, in the context of his assertion that God works all in all, find the ultimate cause in God himself, in his intention, and in his working. This decision is not made by man's supposedly free will, but only by God's willing and working. He chooses some to be saved and he rejects the others without an apparent reason for either choice. He gives faith to one through the working of His Spirit; and he refuses to give faith to others so that they are bound in their unbelief. Salvation and destruction thus result from God's previous decision and his corresponding twofold activity. God's choice is not based on the individual's condition; it establishes this condition. This means an unconditional, eternal predestination both to salvation and to damnation.
Luther does not reach this conclusion on the basis of philosophical speculation about God, but finds it in the Scripture. He experienced it in God's relationship to him personally; and the God whom he thus personally experienced is the very same God who speaks and is proclaimed in the Scripture. Paul especially testified to Luther that God makes this twofold decision and that he hardens those who are lost: "God has mercy upon whomever he wills, and he hardens the heart of whomever he wills" (Rom. 9:18). Paul illustrates this with the picture of the potter making vessels of honor as well as dishonor out of the same clay (Rom. 9: 20 ff.). In addition, Paul quotes Malachi, "Jacob I loved, but Esau I hated" (Rom. 9:13). And Paul specifically refers to God's treatment of Pharaoh (Rom. 9:17)
The position Scripture thus presented to Luther was also the inescapable result of his understanding of God. He even cites man's innate rational concept of God as an additional proof. It seems blasphemous even to think that God does not work man's decision to believe or not to believe, as though God could be surprised by man's choice and men might be saved or lost without God knowing it. Whoever so thinks denies that God is God and makes fun of Him as though he were a ridiculous idol." Whoever speaks seriously of God must necessarily teach his foreknowledge and his unconditional determination of all things.
Luther thus finds a twofold will of God in the Scripture. Together with statements about God's all-inclusive grace are other statements which express another willing and working of God which stands with his willing and working of salvation. Together with grace stands wrath, a wrath which rejects and which is no longer a part of love; and this is found not only in the Old but also in the New Testament. Luther did not draw a two-sided picture of God from his own imagination, but he saw it already present in Scripture. The God of the Bible is not unequivocally the God of the gospel. The God of the Bible is not only the God of all grace but is also the God who, if he wills, hardens and rejects. This God even treats a man equivocally: he offers his grace in the word and yet refuses to give his Spirit to bring about his conversion. He can even harden a man—in all this Luther does not go in substance beyond the difficult passages of Scripture which describe God as hardening a man's heart.
Luther, however, summarized the substance of such scriptural statements in the sharpest possible expressions. In The Bondage of the Will he teaches that God has a double will, even a double reality. The God revealed and preached in the gospel must be distinguished from the hidden God who is not preached, the God who works all things. God's word is not the same as "God himself." God, through his word, approaches man with the mercy which (according to Ezekiel 33) does not seek the death of the sinner but that he turn and live. But the hidden will of God, the will we must fear, "determines for itself which and what sort of men it chooses to enable to participate in this mercy offered through the proclamation." God "does not will the death of the sinner, that is, according to his word; he does, however, will it according to his inscrutable will." God revealed in his word mourns the sinner's death and seeks to save him from it. "God hidden in his majesty, on the other hand, does not mourn the sinner's death, or abrogate it, but works life and death in everything in all. For God has not limited himself to his word but retains his freedom over everything. . . . God does many things that he does not show us through his word. He also wills many things his word does not show us." [Paul Althaus, The Theology of Martin Luther (Philadelphia: Fortress Press, 1966) pp. 274-276].