My Philosophy and Ethics teacher is Atheist - so that's not a problem. What I would say is you'll find it very hard to JUST teach philosophy as in most schools it is Philosophy AND Ethics.
Do your PGCE in RE then teach RE because most RE teachers (you'll have to do KS3 to A Level, sorry) only teach Philosophy OR Ethics at A Level (so students have two teachers).
That's the only real way that you'll be able to do it, i'm afraid.