From the BBC program "The Interview" with Host Owen Bennett Jones.
Jones: You're a religious man.
Collins: That's true.
Jones: And you came to religion quite late.
Collins: Also true. I was not raised in an environment where faith was considered all that relevant. I slipped away from any glimmerings of that by the time I was a grad student in physical chemistry and became a pretty obnoxious athiest actually. And then when I went to medical school I began to realize I had not really done a good job of considering the evidence for and against faith. As a scientist that's something you're not supposed to do; draw conclusions without considering all the evidence. All around me in the hospital were people suffering terrible diseases who's faith in many instances seemed to be the one rock they clung to, and that was curious. I assumed it was some psychological phenomena, but I figured I better learn more about it, and so I began to try to learn more about what faith is all about, and accidently converted myself.
Jones: On a rational basis or just a faith basis?
Collins: On primarily a rational basis. A helpful minister gave me a small book "Mere Christianity" by CS Lewis. CS Lewis was an Oxford scholar who had himself been an atheist who set out to try to undergird his athiesm and accidently converted himself too. As I read his book I realized that my arguments against the plausibility of faith were all those [arguments] that as a school boy you could drive trucks through the flaws in the logic, and he [Lewis] very effectively did so, page after page, reading my mind practically; maybe because he had traveled practically the same path.
Ultimately after a year of reading and considering many other faith options I came to the conclusion that while science and reason cannot prove the existence of God, the existence of a God and a God who cares about human beings, is a lot more plausible than to deny that [existence] on a purely rational basis. If you had to sort of look at the odds, the odds were in favor of God's existence.
Jones: So there's still a leap of faith?
Collins: There always will be. You can't prove the supernatural using the tools of the natural.