First he had read C.S. Lewis. Personally I never liked the material that Lewis wrote specifically for children, but the stuff that he wrote for 'adults' is quite astounding. I have quoted him sometimes before, e.g. here and here, and found some of his writing entirely risible and facile.
The particular passage that precipitated the conversion of chemist Francis Collins appears to have been this one:
"I am trying here to prevent anyone saying the really foolish thing that people often say about Him: “I’m ready to accept Jesus as a great moral teacher, but I don’t accept His claim to be God.” That is one thing we must not say. A man who was merely a man and said the sort of things Jesus said would not be a great moral teacher. He would either be a lunatic—- on a level with the man who says He is a poached egg—- or else He would be the Devil of Hell. You must make your choice. Either this man was, and is, the Son of God: or else a madman or something worse. You can shut Him up for a fool, you can spit at Him and kill Him as a demon; or you can fall at His feet and call him Lord and God. But let us not come with any patronising nonsense about His being a great human teacher. He has not left that open to us. He did not intend to."
I do agree with Lewis in one way. I also believe that he was not a great human teacher . . . but for the reason that I doubt that he ever lived as described in the New Testament.
However, what was the context where this had such an effect? It was the sight of a frozen waterfall.
"Lewis was right. I had to make a choice. A full year had passed since I decided to believe in some sort of God, and now I was being called to account. On a beautiful fall day, as I was hiking in the Cascade Mountains during my first trip west of the Mississippi, the majesty and beauty of God’s creation overwhelmed my resistance. As I rounded a corner and saw a beautiful and unexpected frozen waterfall, hundreds of feet high, I knew the search was over. The next morning, I knelt in the dewy grass as the sun rose and surrendered to Jesus Christ. "
How surprising that a a rational scientist turned his view of life over for a reason such as this! Why did a waterfall lead him to this specific God when there were so many others to choose from?