In about 1534, he underwent a sudden conversion and became an ardent Protestant. He went to Basel, where he wrote and published the first edition of his Institutes of the Christian Religion, a work of systematic theology and a detailed logical account of grace. His doctrine is rooted in the experience of God's grace at work in his own heart, and unwillingness to attribute its presence to anything but the mercy of God, a determination never to claim that he has done anything more or better than Judas Iscariot to deserve a better destiny.
Calvin’s output was immense. In addition to the Institutes (1700 pages) he wrote commentaries on almost all the books of the Bible: many tracts and treatises discussing important theological controversies; hundreds of sermons (342 on Isaiah alone!); and numerous letters. Every Christian, Calvin insisted, must possess a measure of doctrinal sophistication or be at the mercy of every theological ill-wind. Pastors in particular must be provided with the tools needed for life-long study in service of the Word of God.
The list of ailments from which Calvin suffered is enough to make a person wince. Theodore Beza, his successor in Geneva, wrote of him, "A brave spirit was the master of a feeble body." Nevertheless, Calvin persevered throughout his suffering, working in the last, most difficult years, preaching until eight days before his death. Unflappable in his vocation, he finally had to be carried into the pulpit in Geneva in a chair. A remark in the dedication to his Commentary of II Thessalonians says it all: "My ministry... is dearer to me than life."
Calvin penned his last letter to his dearest friend, Guillaume Farel, only days before he died: "It is enough that I live and die for Christ, who is to all his followers a gain both in life and in death." His grave is unmarked.
Schaff recounts the last months of Calvin's life in some detail in his History of the Christian Church, volume 8. Regarding Calvin’s own last wishes, he writes:
Calvin had expressly forbidden all pomp at his funeral and the erection of any monument over his grave. He wished to be buried, like Moses, out of the reach of idolatry. This was consistent with his theology, which humbles man and exalts God.
Beza, however, wrote a suitable epitaph in Latin and French, which he calls "Parentalia" (i.e. offering at the funeral of a father):
"Shall honored Calvin to the dust return,
From whom e’en Virtue’s self might learn;
Shall he -of falling Rome the greatest dread,
By all the good bewailed, and now (tho’ dead)
The terror of the vile - lie in so mean,
So small a tomb, where not his name is seen?
Sweet Modesty, who still by Calvin’s side
Walked while he lived, here laid him when he died.
O happy tomb with such a tenant graced!
O envied marble o’er his ashes placed!"