In my mind, creating a false religion has been along the same lines as creating an evil murderer. Neither of them are good, but that doesn't mean it is sinful for a writer to have them appear in his story. If that makes any sense. Personally, I wouldn't say it's blasphemous or sacrilegious.

All the same, though, I don't go too deep in making one, like I wouldn't spend long periods immersing myself in a wicked murderer's POV. It affects your mind, the things you think about, so I think it's good to be wise about such things. * grimace *

If you want to see how other authors deal with it, most Christian fantasy books these days have some sort of take on religious matters, I believe (though I haven't read all too many of them myself).

I'm sorry I can't tell you of many off the top of my head, I don't tend to read many modern books... But 'A Cast of Stones', the Dragonking trilogy, and all the fantasy published by Enclave Press (formerly Marcher Lord Press), have religious and Christian elements in them.

The thing is, though, that the Narnian Chronicles don't have 'Christianity', they have the otherworldly equivalent of Christianity, which is what most Christian fantasy books I've read tend to do. They don't have a 'Christ' because that's a Greek word and there are no Greeks, and they don't have 'Jehovah' because that's a Hebrew word and there are no Hebrews, but they do have a Christian theology present in the world anyway.