It
is okay to insult any religion or ideology you want to bash or make fun of. The only difference is, Christians won't refer to you as a "racist" when you criticize their ideology.
They just happen to be slightly more open-minded, sane, and more often than not refrain from bombing and assassinating newspapers/journalists who make fun of their religion.
Trashing religion is no different from bashing communism, for example. Both are a set of ideologies and personal beliefs.