I'm not sure I can help with the cognitive dissonance. I can suggest that you do as GeofferyF and suppose that Christianity is true in the universe that Narnia inhabits, even if you don't believe it exists in this universe.
I can also say that true things are rarely easy to comprehend, not if they have real meaning.
I can't help you with feeling as if you are falsely proselytizing if you write Aslan as Christ. The thing is, many if not most of Aslan's attributes are Christ's: if you write Aslan as Aslan, and stay true to canon, you fall into that trap regardless because, just as many people read Aslan in CoN as Christ, they'll read your story and, if Aslan is written like Aslan, they'll see it in your fic as well.
Re: REALLY long comment, part 3
I can also say that true things are rarely easy to comprehend, not if they have real meaning.
I can't help you with feeling as if you are falsely proselytizing if you write Aslan as Christ. The thing is, many if not most of Aslan's attributes are Christ's: if you write Aslan as Aslan, and stay true to canon, you fall into that trap regardless because, just as many people read Aslan in CoN as Christ, they'll read your story and, if Aslan is written like Aslan, they'll see it in your fic as well.