Dictionary.com defines the noun Christian as a person who believes in Jesus Christ; a Christ Follower. But to truly be Christ-followers we mustn't simply believe that Jesus existed, we must believe that he was who he said he was. CS Lewis famously addresses the contradiction of the strictly "moral" Jesus in Mere Christianity:
"A man who said the sort of things Jesus said would not be a great moral teacher. He would either be a lunatic — on a level with the man who says he is a poached egg — or else he would be the Devil of Hell. You must make your choice. Either this man was, and is, the Son of God: or else a madman or something worse."
Certain Atheists who acknowledge Christ's existence but don't believe he was the son of God have criticized Lewis for forcing an "either/or" scenario upon his readers: "[Lewis] habitually confronts his readers with the alleged necessity of choosing between two alternatives when there are in fact other options to be considered." -John Beversluis
Yet, the boldness of a man claiming to be the son of God lends credibility to Lewis' point. Jesus wasn't a carpenter claiming to be a poet or a peasant claiming to be a king, he was a man, who, accompanied by signs and wonders, laid claim as the Son of God.
The magnitude of this is so striking that Lewis' "either/or" becomes unquestionably reasonable, and it is Jesus' very claim itself, accompanied by the belief that he existed, died, and rose again to reconcile the world to himself, that the entire Christian faith is founded upon. It is the fundamental, non-negotiable core, and it puts Christianity as simply another religion to death along with the idea that any number of good works can save us.
"For by grace you have been saved through faith. And this is not your own doing; it is the gift of God," [Ephesians 2:8].
Dietrich Bonhoeffer perhaps sums up true Christianity best, without the constraints of tradition or religious requirement in Letters and Papers from Prison: "To be a Christian does not mean to be religious in a particular way, to make something of oneself (a sinner, a penitent, or a saint) on the basis of some method or other, but to be a man-- not a type of man, but the man that Christ creates in us. It is not the religious act that makes the Christian, but participation in the sufferings of God in the secular life."
Finally, as CS Lewis points out: “The New Testament talks about men “being born again”, “putting on Christ”, “having the mind of Christ”. Put right out of your head the idea that these are only fancy ways of saying that Christians are to read what Christ said and try to carry it out – as a man may read what Plato or Marx said and try to carry it out. They mean something much more than that. They mean that a real Person, Christ, here and now, in that very room where you are saying your prayers, is doing things to you. It is not a question of a good man who died two thousand years ago. It is a living Man, still as much a man as you, and still as much God as when He created the world, really coming and interfering with your very self; killing the old natural self in you and replacing it with the kind of self He has. At first, only for moments. Then for longer periods. Finally, if all goes well, turning you permanently into a different sort of thing; into a new little Christ, a being which, in its own small way, has the same kind of life as God; which shares in His power, joy, knowledge, and eternity.”
Audio (.mp3)