Calvin's Only Letter to Luther

There is only one letter that John Calvin wrote to Martin Luther, and Luther never read it.

A helpful account of its circumstances can be found in Phillip Schaff's History of the Christian Church Vol. 8, pp. 610-614. Schaff refers to these circumstances as "Calvin and the Nicodemites."

A great practical difficulty presented itself to the Protestants in France, where they were in constant danger of persecution. They could not emigrate en masse, nor live in peace at home, without concealing or denying their convictions. A large number were Protestants at heart, but outwardly conformed to the Roman Church. They excused their conduct by the example of Nicodemus, the Jewish Rabbi, who came to Jesus by night.Calvin, therefore, called them "Nicodemites."

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The Nicodemites charged Calvin with immoderate austerity. "Away with this Calvin! he is too impolite. He would reduce us to beggary, and lead us directly to the stake. Let him content himself with his own lot, and leave us in peace; or, let him come to us and show us how to behave.

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The French Protestants were under the impression that Luther and Melanchthon had milder and more practicable views on this subject, and requested Calvin to proceed to Saxony for a personal conference.

Calvin didn't go to Wittenberg. Rather, he wrote to Melanchthon and Luther on January 20, 1545. Calvin's letter to Melanchthon can be found here.  Calvin seemed confident that Melanchthon would look the situation over fairly, despite their disagreements. Of the letter to Luther though, Calvin states:

With regard to Dr. Martin there will be somewhat more of difficulty. For so far as I could understand by report, and by letters from different persons, the scarcely pacified temper of the man might, on very slight occasion, break out into a sore. On that account, therefore the messenger will shew you the letter which I have written to him, that on examination of the contents, you may proceed as you think advisable, that nothing may be attempted therein either rashly or unadvisedly, which may hereafter produce unpleasant consequences. I am aware that you will do all that you can worthily accomplish to the utmost of your power, in every thing seemly and befitting.

Luther never saw the letter. Melanchthon  replied back:  " 'I have not shown your letter to Dr. Martin,' he replied to Calvin, April 17, 1545, 'for he takes many things suspiciously, and does not like his answers to questions of the kind you have proposed to him, to be carried round and handed from one to another.' " Schaff points out,

Melanchthon substantially agreed with Calvin; he asserts the duty of the Christian to worship God alone (Matt. 4:10), to flee from idols (1 John 5 : 21), and to profess Christ openly before men (Matt. 10: 83); but he took a somewhat milder view as regards compliance with mere ceremonies and non-essentials. Bucer and Peter Martyr agreed with this opinion. The latter refers to the conduct of the early disciples, who, while holding worship in private houses, still continued to visit the temple until they were driven out.

LETTER 124. TO LUTHER.
CALVIN SUBMITS TO LUTHER SEVERAL OF HIS WRITINGS, OF WHICH HE DESIRES TO OBTAIN HIS APPROBATION.

January 21, 1545.
To the very excellent pastor of the Christian Church, Dr. M. Luther, my much respected father,

When I saw that my French fellow-countrymen, as many of them as had been brought out from the darkness of the Papacy to soundness of the faith, had altered nothing as to their public profession, and that they continued to defile themselves with the sacrilegious worship of the Papists, as if they had never tasted the savor of true doctrine, I was altogether unable to restrain myself from reproving so great sloth and negligence, in the way that I thought it deserved. How, indeed, can this faith, which lies buried in the heart within, do otherwise than break forth in the confession of the faith? What kind of religion can that be, which lies submerged under seeming idolatry? I do not undertake, however, to handle the argument here, because I have done so at large already in two little tractates, wherein, if it shall not be troublesome to you to glance over them, you will more clearly perceive both what I think, and the reasons which have compelled me to form that opinion. By the reading of them, indeed, some of our people, while hitherto they were fast asleep in a false security, having been awakened, have begun to consider what they ought to do. But because it is difficult either casting aside all consideration of self, to expose their lives to danger, or having roused the displeasure of mankind, to encounter the hatred of the world, or having abandoned their prospects at home in their native land, to enter upon a life of voluntary exile, they are withheld or kept back by these difficulties from coming to a settled determination. They put forth other reasons, however, and those somewhat specious, whereby one may perceive that they only seek to find some sort of pretext or other. In these circumstances, because they hang somehow in suspense, they are desirous to hear your opinion, which as they do deservedly hold in reverence, so it shall serve greatly to confirm them. They have therefore requested me, that I would undertake to send a trusty messenger to you, who might report your answer to us upon this question. And because I thought it was of very great consequence for them to have the benefit of your authority, that they might not fluctuate thus continually, and I myself stood besides in need of it, I was unwilling to refuse what they required. Now, therefore, much respected father in the Lord, I beseech you by Christ, that you will not grudge to take the trouble for their sake and mine, first, that you would peruse the epistle written in their name, and my little books, cursorily and at leisure hours, or that you would request some one to take the trouble of reading, and report the substance of them to you. Lastly, that you would write back your opinion in a few words. Indeed, I am unwilling to give you this trouble in the midst of so many weighty and various employments; but such is your sense of justice, that you cannot suppose me to have done this unless compelled by the necessity of the case; I therefore trust that you will pardon me. Would that I could fly to you, that I might even for a few hours enjoy the happiness of your society; for I would prefer, and it would be far better, not only upon this question, but also about others, to converse personally with yourself; but seeing that it is not granted to us on earth, I hope that shortly it will come to pass in the kingdom of God.

Adieu, most renowned sir, most distinguished minister of Christ, and my ever-honored father. The Lord himself rule and direct you by his own Spirit, that you may persevere even unto the end, for the common benefit and good of his ownChurch.
— Yours, John Calvin.