In Mere Christianity, C.S. Lewis backs into his topic. He does not, in the first chapter, outline what he thinks Christianity is but rather he begins with where he thinks we all are, namely that we all have a basic understanding of the moral law. Plainly put, we all walk around with some idea in our head that people ought to be treated some certain way. I don't steal from you and you ought not steal from me, because there is this basic rule we ought to apply to one another as humans.
Lewis notes that where there's a rule or law when it comes to human affairs, that rule or law can and will be broken. So it goes with this basic moral law that we all have knocking around in our heads.
At this point, it'd be hard to see how Lewis's argument can be doubted. This is precisely because the point of the argument is so simple. Yet simple truths like this one are profound, in a way--the point being that whatever our religion, whether we have one or not, we still have this primitive morality innate to us, and we know we ought not violate it.
We're off and running with Lewis's view on Christianity, but we have a long way to get from this moral law to what Christianity is.