As an atheist, I know exactly what you mean. But my point is that Catherine continues to believe and respect him after what the cultist that Catherine spoke to said. From what the cultist told Catherine, we learn that everyone who dies will not go to Heavenly dimensions, and those who do not follow the teachings will go to hell dimensions. Also, the teachings must contradict the true love that Catherine believes in. Because Cultist wants to take away from him the woman his own brother is in love with, and he says he has a vision for it. But when his brother opposes it and dies, Cultist says he will suffer in a hellish dimension. It's illogical that Catherine didn't realize the contradiction here and approved of the cultist.
Just like people continue to believe after their priest rapes their child, or whatever. It's the whole 'men are fallible, but I still believe the parts that make me happy' bit that so many use to excuse so many atrocities.
It is much harder to correct a lie, than it is to spread one. Especially, when people have their whole identity wrapped up in it. They want to believe so much, that they will easily overlook things they know are a problem, and just stick their fingers in their ears. They will continue to make believe, and the more you push, the more they will double down, ignoring anything that makes them uncomfortable.
That shell is a fragile thing, and some never get passed the denial and anger stages, lashing out at anyone who challenges their foundation. We are lucky she was
willing to even come home after that, it could have gone a lot worse. Been there, done that. That is part of why I didn't like his wording there, but it was not something he wanted to change, particularly, since they have their own religion in the wastes, so he can't object on those grounds.