C.S. Lewis said that one of the reasons that he believes Christianity is that it is not the sort of thing anyone would have made up. In Mere Christianity, he wrote:
Reality, in fact, is usually something you could not have guessed. That is one of the reasons I believe Christianity. It is a religion that you could not have guessed. If it offered us just the kind of universe we had always expected, I should feel we were making it up. But, in fact, it is not the sort of thing anyone would have made up. It has just that queer twist about it that real things have. So let us leave behind all thee boys’ philosophies – – these over simple answers. The problem is not simple and the answer is not going to be simple either. (C.S. Lewis, Mere Christianity, Book II, chapter 2, “The Invasion,” page 36).
C.S. Lewis made a similar point in The Silver Chair through his character Puddleglum. When Puddleglum is giving his apologetic to the Queen of the Underland he says:
“One word, Ma’am,” he said, coming back from the fire; limping, because of the pain. “One word. All you’ve been saying is quite right, I shouldn’t wonder. I’m a chap who always liked to know the worst and then put the best face I can on it. But there’s one more thing to be said, even so. Suppose we have only dreamed, or made up, all those things – -trees and grass and sun and moon and Aslan himself. Suppose we have. Then all I can say is that, in that case, the made-up seem a good deal more real than the real ones. Suppose this black pit of a kingdom of yours is the only world. Well, it strikes me as a pretty poor one. And that’s a funny thing when you come to think of it. We’re just babies making up a game if you’re right. But four babies playing a game can make a play-world that licks your real world hollow. That’s why I’m going to stand by the play world. I’m on Aslan’s side even if there isn’t any Aslan to lead it. I’m going to live on like a Narnian as I long as I can even if there isn’t any Narnia.”
Nowhere is Lewis’ observation more true than with grace. Made up religions always rely, one way or another, on works. There is something which must be done in order to achieve the goal. But the idea that salvation is a gift from God (Ephesians 2:8-9) is the heart of the Christian message and not the sort of thing anyone would have made up.