C.S. Lewis during World War II was asked by the BBC to do some radio programs about the fundamentals of Christianity. He started these 15 minutes broadcasts in 1941 and they came together in part as the book "Mere Christianity", which published in 1952. It is a timeless classic that is as influential today as when it was originally written by Lewis.
The book takes us from when Lewis was an Atheist through as his ultimate conversion to Christianity. The book looks closely at what it calls moral law and says that understanding moral law is basic. His sense of humanity's failure to keep the moral law is key to how he sees Christ's work in overcoming the effect of Satan's rebellion.
It gives a comparison to other views of what God is. He states that Christ either was God or was not or just thought he was, and then based on Christ's character, he concludes that he was. The books looks at how Christian beliefs result in creditable ethics.
A basic look at Christianity that serves as a fundamental touchstone for Christian discourse.