If you have one group of people whose goal is to rule the world and another whose goal is to go to heaven by following instructions written in a book by the first group, which group wins in this world?
King Clovis and Charlemagne probably thought they were doing a good thing by forcefully converting the Franks to Catholicism. Their former religions worshiped their ancestors and nature, and some believed they would only go to heaven if they died in battle or childbirth. It did help unite the Germanic people, but the long term effects are now being seen.