How the Romans handled rival religions
A hadith attributed to Muhammad said that there would be 73 sects of Islam (of which only one would reach heaven). However many there are, they seem as likely to kill each other as they do the infidel. The one virtue of classical religion was that it embraced all gods, come what may. So when the Romans arrived in Britain, they put up altars to local gods as well as their own. Olympus knows what Zeus and Mercury made of Garmangabi, Gontrebi, Mogti and Ricagumbeda, but that was their problem. Indeed, Minucius Felix (3rd c. ad) pointed out that one reason why Romans were so successful was that the moment they defeated a people, they sacrificed to that people’s gods as well. The big exceptions Romans came across were, of course, Jews and Christians.