Many role-players comes from educated societies where, according to statistics, subjects such as religion, slavery, sex and many other topics are considered taboo. This is often transferred in to the roleplaying game. But this is where a player have to put their own norms aside, and consider what is normal for their characters. Though a player might want to explain to the other players, specially incase that he is playing in a new group, that his characters opinions are not related to his own.
Characters that is living in harsh, war-torn areas the people around the areas might take prisoners of war as slaves, and when it get really harsh they sacrifice the slaves to the gods. Where most people don’t think well about both slavery and sacrificing people, not to talk about the decreasing religious beliefs, to keep a character in its role, a player might choose to think that both slavery and sacrifices are absolutely normal, even though this will clash with his own conviction.
A trend that is going on is that players does not select a religion for their characters either as agnostics or as atheists, which is difficult to maintain in a world where gods, avatars and celestials both communicate directly to humans and proves their power through clerical divines, giving humanoids the healing power through their faith.
A game master might choose to either decrease the healing power to a non-believer, or to entirely remove the effect of the spell, as a way for the gods to punish the person who decided not to believe in them.
Trying to relate more to the region your character is coming from, would help relate more to who your character are, meaning a group of 5 adventurers living in a world where religion is an everyday thing and gods are present, going around as non-believers just don’t add to the realism.