Hot take, apparently, but genuinely harassing or insulting anyone's religion is not ok.
Criticizing religious institutions, proselytizing, extremism, or horrible behaviors/beliefs done in the name of a religion? YES! Totally. That should be criticized.
Unpromptedly popping up on random posts by religious folks to say anything along the lines of "god isn't real", "the pagan gods are fake", "there is only one true religion", "[any religious group] are stupid/dangerous/barbaric" -- NO.
It's rude. It's unnecessary. And sometimes, especially in the case of minorities or oppressed groups, it's outright hateful. Theres nothing helpful, funny, or cool about randomly insulting one of the most personal aspects of a persons life. Unless it's invited or warranted, stfu.