The equanimity of his religous patients in the face of cruel fate fascinated him
He was struck by the arguments in C.S. Lewis' Mere Christianity (i.e., "Christians can make rational arguments!").
An epiphany as he was hiking through the Cascade mountains.
This is an area where you have to be careful to distinguish between the reasons people sincerely believe they believe in x, and the real causes at work here. Psychologists have long known that people can be primed toward particular preferences through manipulating environmental inputs, but when asked why they made the choices they made most humans can sincerely offer you a complex and plausible sounding rationale. I believe that religious belief at any moment in time is shaped by a large number of factors. But, many people will claim that one particular "reason" is the root of their belief, whether it be reading the Bible, the Cosmological Argument or a personal "experience" with the divine. I do not doubt the sincerity of these assertions, the problem with examining these beliefs about religion though is importance that individuals will ascribe to their assertions. This was, I believe, the issue that John Derbyshire was getting at in his review of Ramesh Ponnuru's new book when he stated "It would be an astounding thing, just from a statistical point of view, if, after conducting a rigorous open-ended inquiry from philosophical first principles, our author came to conclusions precisely congruent with the dogmas of the church in which he himself is a communicant."