The Roman Empire represents a fascinating picture of a trully cosmopolitanism and multicultural society.  Citizens and non-citizens alike were free to worship as they liked as long as they also maintained their obligations to the ancient gods of Rome.   So a citizen living in Britain might be a devoted member of the cult of Britannia and was able to worship accordingly as long as he continued his veneration of Juno.  The ancient religion was of Greek origin and took the form of a corporate veneration.  All must join together in veneration and festivals as a corporate whole and an atheist could bring down the wrath of the gods on the community.  The ancient philosophers who doubted the existence of the gods were careful to maintain their observation because they did not doubt the opinion of their peers.  This veneration was not a onerous duty, involving dropping a single pinch of incense onto an altar.  Members of the Sacralogus (Christians) refused to do this, simple and meaningless duty, an action which the great mass of Roman citizens found incredibly odd.  By refusing to join in with their peers the Roman Christians put themselves outside of citizenship which explains why they could be executed in the arena, a fate that they would normally escape because of their status.  Whilst Jews and Christians found themselves at odds with Roman society, because of the absolutist and exclusionist nature of their religion, many other cults were happy to share devotion with the ancient powers.