I am against Islam.
we need to break this down a bit more man
everyone hates religious extremists, muslim, christian or otherwise...
there is a massive difference (i damn well hope anyways lol) between your parents, who are christian, and jeffery dahmer, also a christian. somebodys religion doesn't make them nuts, it's all the other crap that goes along with it... social standing, family life and umpteen other things.
even your form of christianity (making a bold leap here because i know you live in the UK) differs from most of the other forms around the world, iirc your religious leaders are in the church of england, not the vatican.
anyways. maybe just say "i am against islamic nutjobs" because that is probably more accurate, or even better "i am against people who use religion as an excuse to do messed up stuff."
and hell, if you want to be against all of islam, go for it, but the bottom line is that the only part of islam that is actually against you
personally are the crazy dudes with neckbeards and c4 vests.
just at a guess i would say that over the entire course of history, christianity has caused more death and societal turmoil than islam (but in the last two centuries no)