Alone these 'heretics' are not a threat, but as soon as two get together it's no longer about one lunatic deviant who you can ignore, but a social phenomenon.
The more you have heretics the stronger their social proof field becomes, enticing people away.
Religions are much more understandable as social movement and political constructs that lean heavily on propaganda rather than as methods of investigation into the nature of reality.
Organised religion is about control. Heresy was just as serious as blasphemy - both could get you executed. Can't have people saying anything against the church, they might start to question their tithes.
They know they won’t change anyone’s mind. But the action of standing up for their beliefs makes them feel like part of something. Part of their tribe.
They can't understand either