For when you get down to it, is not the popular idea of Christianity simply this: that Jesus Christ was a great moral teacher and that if only we took His advice we might be able to establish a better social order and avoid another war? Now, mind you, that is quite true. But it tells you much less than the whole truth about Christianity and it has no practical importance at all.
It is quite true that if we took Christ’s advice we should soon be living in a happier world. You need not even go as far as Christ. If we did all that Plato or Aristotle or Confucius told us, we should get on a great deal better than we do. And so what? We never have followed the advice of the great teachers. Why are we likely to begin now? Why are we more likely to follow Christ than any of the others? Because He is the best moral teacher? But that makes it even less likely that we shall follow Him. If we cannot take the elementary lessons, is it likely we are going to take the most advanced one? If Christianity only means one more bit of good advice, then Christianity is of no importance. There has been no lack of good advice for the last four thousand years. A bit more makes no difference.
But as soon as you look at any real Christian writings, you find that they are talking about something quite different from this popular religion. They say that Christ is the Son of God (whatever that means). They say that those who give Him their confidence can also become Sons of God (whatever that means). They say that His death saved us from our sins (whatever that means).
There is no good complaining that these statements are difficult. Christianity claims to be telling us about another world, about something behind the world we can touch and hear and see. You may think the claim false, but if it were true, what it tells us would be bound to be difficult—at least as difficult as modern Physics, and for the same reason.
C.S. Lewis, Mere Christianity (1952; Harper Collins: 2001) 155-156.