I'm a Catholic; have been my whole life. This book, while complete fiction, is a humorous and heartfelt way for me to fill in the "missing" years in the life of Jesus. I love it so much that I re-read it every year during Lent before Easter.
The entire HP series is basically tattooed on my soul but this one... This is the first book I can remember causing me actual physical pain. The sheer joy I felt when Harry had his chance at a real family with Sirius and then the absolutely heartache when that was taken away from him (and in OotP even more so!) It's the kind of soul crushing that stays with you forever. I get little heart squeezes when I think about this book.
The first time I read this was, I think, in eighth grade and it changed my life. This book paints a completely realistic picture of the hardships endured by everyday kids. You really feel Charlie's joy, anxiety, pain... I made all my friends read this book.