It's "Someone is wrong on the internet" syndrome, only much worse because it's "Someone I know is wrong right next to me", which makes it much stronger.

ETA: Also, it's just that atheism seems so obvious to me that anyone with a shred of sense should easily be able to see that Christianity is false, just by praying and seeing what happens. I know intellectually that religious people aren't actually idiots or insane people, but I feel like they are.