Religion is probably one of my least liked things the world, but seeing the same old tried and true "evil church" trope with literally no other depth is kinda boring. If your enemy is actually bad, there's no point in making him a caricature or straw-man; ironically, you should give the devil his due. Slime is better than most isekai, but along with the general childishness, the tropes like this or the "game-like" world kinda hurt it.