They had a reason to: to control others.
If I'm some medieval European ruler, it's an advantage for me to follow a religion where I can use that religion to argue that I'm chosen by God and all my opposition are heretics who'll be going to hell for disobeying God. It's also an advantage for me to get as many people in my country as possible to follow said religion, since it means more people will believe my claims and won't dare oppose me.



