I hate that this story has existed for two years, and I have only just now found it. I rarely read any fanfic these days, as adulthood and life have (justly) left no room for it, but tonight I had nothing on my plate, and I wandered over to Painless J's for old times sake.
Exegesis. The concept has always appealed to me; understanding of scripture based not just on what is written, but on what is behind what is written. As someone who was raised in the Greek Orthodox church, and who attended Catholic school for 9 years, and who today could best describe herself as a self-guided, unassuming, and often superstititious believer, the concept is really the cornerstone of how I continue to operate within my own little corner of the faith.
While I always enjoy some good Snarry romance, the pairing was secondary, (although the smut was both well-earned and well-written) to the world you created. From the Giles (sqee!) reference to the recurrence of Thai food to the Boston setting to the reverential and obviously personally informed descriptions of the church in Romania, this story was tailor-made (almost frighteningly so) for me. I have never sought out religious themes in fanfiction, and after reading this I never plan to. Because I know I'll never find better.
I'll be honest, I wasn't expecting an AU when I clicked on the link, but I was already hooked after the first few paragraphs so I suppose that doesn't matter so much. Your writing was clear and insightful, and although I felt some of the Vampire-lore dragging on a bit (specifically Voldemort's speech before turning Dumbledore, when I felt he was playing a bit of the Bond villian, gabbing on when he should have been... well, killing) I actually quite enjoyed the historical and genealogical aspects to it. The only thing I would have changed had I been beta-ing, would be that one word paragraph. "Pansy." It was unneseccary. We all knew who she was, she wasn't all that important, and she was about to die anyway. But one word out of 49 thousand or so...I'd say that makes this story just about the best I've read.
Honestly, change the names of the characters, get rid of the "Lumos" and the owl-post references, and this is something that wouldn't be out of place next to Oates and Bradbury in an anthology of modern short stories.