I had a friend who was culturally Catholic, but ended up a born-again Christian. It affected our friendship, and possibly still does, although we are still friends. Whilst he might have his own way of describing this, I guess I'd make the following points from my personal, atheistic viewpoint, since I think it's worth seeing these things from 'both sides', so to speak;
1) His faith in and of itself never worried me too much. I have had some very Christian friends, I guess you could say. I don't care if you believe in God.
2) He was so excited in his new found faith he spoke about it a lot. I'm okay with that too, as long as you're listening as well as speaking. Otherwise we're not conversing...you're monologuing. Quite apart from having a different viewpoint, I hate being talked at.
3) There were specific issues where his opinion changed rapidly. Almost literally overnight. So homosexuality become a dangerous topic, as did discussions on whether there should be football games on Easter Friday, or about how we should determine refugee intakes. Age of the Earth was an interesting topic which came up once. He claimed humans and dinosaurs lived together, to which I asked him why he believed that. It wasn't a particularly fruitful discussion. He might see that as me being anti-faith, and in a sense he's right, but I'm certainly not anti-Christian. Pre-born again a natural response to any such claim would have been 'Why do you think that?' but once he was born again, such questions were more readily seen as a challenge to his faith (which...again...in a sense they are, depending on how you define 'faith')
His faith changed his views on things. Anything which changes your views will impact on friendships, and the more rapid the change the more potential change it involves. That isn't to argue people should suppress their faith or whatever. It just is.