John Maynard Smith was a renowned British theoretical evolutionary biologist, geneticist, and science author. He was instrumental in the application of game theory to evolution and theorized on other problems such as the evolution of sex and signaling theory. 
The following is from a Humanist News interview, which took place in 2001:
You became an atheist whilst quite young - can you talk about this first major transition in your thinking?
I was brought up in the Church of England, which is more of a social club than a religion, and believed in Christianity until I was about 14 or 15 when, largely as a result of learning about evolution and Darwin , I started having doubts. The really decisive moment occurred at school. I was at Eton , which was a really odious school then, snobbish and anti-intellectual - although not universally so, as there were some excellent masters, and I was taught mathematics brilliantly - but the boy culture of my youth was elitist. They considered themselves to be a separate breed. By the time I was 15 I had come to greatly dislike this. The one person my schoolmasters really hated was J. B. S. Haldane - because he was an atheist, a socialist, a republican and divorced - and so I thought I had better read this guy. To their credit, his essays were in the school library, so I took out a book called Possible Worlds , and it completely blew my mind. His mixture of extreme rational science, blasphemy and imagination, was a way of thinking that I had never met before.
I remember sitting there as a boy of 15 reading the title essay, and that was it! It got me interested in evolution and alternatives to religious interpretations of the world, but it was also enormously encouraging to know there were people like that in the world because up until then I had been exposed to ideas that didn’t very much appeal to me. Of course, twenty years later I was to work with Haldane, but a lot was to happen before that.