Recent research found that damage to specific brain networks, primarily in the right hemisphere, is linked to higher levels of religious fundamentalism. These brain regions are involved in cognitive functions like reasoning, belief formation, and moral decision-making.
We've know that religious fundamentalism is a mental illness for a very long time already. Being religious in and of itself isn't the problem. It's when you insist everyone else has to do what you want.
I believe the key difference is humility vs arrogance. The humble approach to religion is to acknowledge that religion is purely hope and that no one knows what happens when we die because there is zero evidence of a soul, or God, or heaven. But the arrogant approach to religion claims to have exclusive knowledge without any proof. They turn religion into an abusive cult where they speak for God and everyone else must get in line without questioning (needing evidence.)