I've known this guy since I was five. Used to be my best friend, but strained schedules caused us to hang out and talk less and less over the years. I recently got back together with him to hang out and play video games or whatever. Anyway, at one point he asked if i had a girlfriend. Because I wasn't sure how he'd react to me suddenly coming out to him, I simply said I wasn't into dating. He responded with the phrase "Please don't be a fag". When I asked him why he was saying stuff like that, he just cited christianity. Should I hide the fact that I'm gay and hope to god he doesn't find out about my boyfriend, or just let him go and come out to him?
Hang out with him more and learn more about him. Ask yourself: Is he the same guy you befriended 10 years ago? If you don't like what he has become, just come out to him and let him go. Citing Christianity as an excuse to be homophobic is stupid, by the way. I doubt you'll want people like that in your life. However, if you feel that this friendship is still precious to you, don't come out yet. Assuming that he's also 15 years old, he could be just immature and brainwashed by an ultra-religous upbringing. But if he's a true homophobe at heart and if you can't picture him accepting you, let him go. Take your time to think this over.