Surprised by Joy

I’m currently reading the book Surprised by Joy, the autobiography of C.S. Lewis. I found this old copy from 1969 in a charity shop here in Dublin.

I heard a voice that cried,
Balder the beautiful
Is dead, is dead—

Even thought I’m agnostic myself, I’m truly fascinated by the debate between atheists and believers. What made Lewis an atheist? Why did he convert to Christianity later in life? I hope that his answers help shed some light on my own thoughts about the matter.