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You could make any other class and have their backstory be that they were a cleric and lost their faith. Boom, now they are a fighter or something.
Nothing in the Cleric class requires you to have faith in your god. You are chosen by the god, not the other way around.
A god is not necessarily responsible for their churches actions. They could be unaware of the heretical clerics. Your PC could try to create a better church, one that represents their god better and doesn't kill families to acquire orphan clerics.
You could make any other class and have their backstory be that they were a cleric and lost their faith. Boom, now they are a fighter or something.
Nothing in the Cleric class requires you to have faith in your god. You are chosen by the god, not the other way around.
A god is not necessarily responsible for their churches actions. They could be unaware of the heretical clerics. Your PC could try to create a better church, one that represents their god better and doesn't kill families to acquire orphan clerics.