In my opinion, there many reasons for this. First one I can think of is they want everyone to have one belief, which is nearly impossible since each and everyone of us is different even we're in the same religion. As a result, the people getting forced have to fight back, which is the second reason, they fight back for what they believe. And last that I can think of, They have to deliver their agenda. Religions have different agenda, who knows maybe they also have hidden ones. In order to implement, they need to do some extreme stuffs.