It's one of most original yaoi I've read. Of course, the religious theme has a big part in it. I consider myself agnostic, and I enjoyed the questions this story brings up.
I have to say that's what I enjoyed the most in this story, like how one can be blessed when loved, or that it never leads nowhere to follow blindly any rules, and that you're creating the world you live in by your beliefs. These questions are very well introduced and treated, the trials the characters are facing really brings you to question yourself. It's rare to see such a philosophical quality in a manga.
But besides that, the themes it gets onto are so deep and complex, that it would have needed some more chapters (or volume) to feel like everything's been treated, at least for the story to feel thoroughly explained. And the end, despite being really good, felt just a bit rushed to my taste.
Even if I finally believed in their love, it didn't seem real at the beginning as there's not really any reason for them to sacrifive for each other, I didn't understand why they were suddenly so attached to each other. Though the only one sex scene comes at the perfect time in the story, as it's a very important moment, I'd have liked it to be a bit longer and detailed. But the characters, and not only the protagonists, are well developped, and I easily get attached to each of them.
Well, it's very good story if you don't mind religious theme which questions the very existence of God. I actually liked the evolution of the characters and the questions they're facing more than the love story in itself. And of course, I loved the art which is simply gorgeous.