The account of Calvin’s conversion is brief and many scholars have sought to pin down exactly how it happened. We know very little. But most scholars agree that Calvin converted to faith somewhere between 1529 and 1530 while studying law at the University of Bourges. T.H.L Parker captures the conversion well:
At some time in 1529 or early 1530 Calvin was converted. We do not know the circumstances…Calvin’s life had similarly been changed and ordered by God’s secret providence. He had been raised out of his obscure origins and made a minister of the Gospel. And this is how the decisive change came about. The plans in which he was destined for the priesthood were suddenly altered and he was put to the study of the law. This is the first disruption in his course, a quite normal, human change of plans. There now occurred a second disruption, and this a divine intervention in his life, overturning the second plan also. He was like a horse, led along by his father in a certain direction, unaware that he had a Rider on his back and a curbed bit in his mouth, until he knew himself pulled up short and turned on a fresh course. This was Calvin’s Conversion. 
Parker reveals much of Calvin’s conversion from Calvin’s own writing in his preface to his commentary on the Psalms. But it is brief. We do not know of a human agent that evangelized to him. His conversion has simply been described as a miraculous encounter with the will of God. And isn’t this the fundamental essence of any conversion? God spoke to him through the Gospel as revealed to him in the Greek New Testament. Calvin was captivated by the will of God, now Calvin’s sole authority in life.
The Apostle Paul, Aurelius Augustine, and Martin Luther all had profound conversion experiences because of the material that is written capturing the conversion itself. We have little for Calvin. I do not believe this is a coincidence. Calvin simply longs for us to know that it was not by human will and coercion that he became a Christian, but solely by the will and grace of God Himself as He was encountered in the Greek New Testament. Behind every human agent that may have aided in our conversion is a God whose sovereign will conducts everything. This is what Calvin wants to highlight, the sovereign goodness of God in his miraculous conversion. It is to God, not men, that we should always be thankful for our awakening to faith. I am thankful for the means, however obscure, that God used to bring Calvin to faith. I have known more of God’s grace, love and sovereign goodness through the writings of Calvin than any other theologian. I praise you Lord for calling Calvin by name, by claiming his mind and heart as ones devoted to you.
1. T.H.L Parker, John Calvin: A Biography (Philadelphia: Westminster Press, 1975), 22. I would highly recommend Parker’s text as a good introduction to the life of John Calvin. It is not too long (less than 200 pages) and it is highly readable. This is the first book I would recommend to anyone looking to go deeper into Calvin’s life.