I agree with another commenter who said that C.S. Lewis is likely more well-known for the Chronicles of Narnia, at least in the broader secular sense. He was a

I agree with another commenter who said that C.S. Lewis is likely more well-known for the Chronicles of Narnia, at least in the broader secular sense. He was a talented writer. That being said, Mere Christianity is pretty clearly preaching to the choir. It is a Christian apologetics book, and the arguments put forward are all problematic. Some of the more notable arguments are so bad that they don't typically get brought up by Christians who regularly debate atheists, like the trilemma he popularized. The book is well-written in the sense that in the hands of a believing Christian who is not likely to approach it critically, it can become an influential source that helps the Christian justify their beliefs to themselves.