Ancient Britain was a melting pot of peoples and ways of life. The natives were mostly Celts who had migrated across the Channel from France or were Scots who had come from Ireland. The intruders were Romans who had almost swum across the Channel in hopes of expanding their frontiers. One potential source of conflict was the differing religions forced by circumstance (namely, the Island was only so big) to abide each other side-by-side for hundreds of years. Granted, the religious struggles of this part of the world in this part of the past had none of the divisiveness of, say, the Reformation; but always there in the struggles between Roman and Celt, Briton and Pict, was the issue of whom or what to worship and how. (Why seems never to have been a question.)
The Celts had their share of religious practices, namely those set out by the Druids. Celts worshipped trees and nature and ravens and symbols of the harvest; they worshipped the god(s) of the hunt and the crops and the land and the sea; they had what we would call bizarre rituals that they believed endeared themselves to their gods. This is not to say that all Celts worshipped alike; indeed, the beliefs changed from tribe to tribe, usually. However, the underlying philosophy was the same.
The Romans, on the other hand, brought their own pantheon with them. Jupiter, Ceres, Mars, Mercury–all were worshipped by Romans determined to protect their souls and their crops, their weapons and their trade. Did the Romans want to conquer the Celts because they didnt understand the religion? This would be a difficult assertion to prove, for the misunderstanding was there–on both sides–but it probably was not one of the overriding reasons for the Roman conquest of Britain. Still, one wonders at the religious tolerance Rome showed its conquered peoples. This type of tolerance seemed to fit in with the other phases of occupation: Peoples could continue their businesses, traditions, and ways of life–as long as they marched in step when Rome told them to.
Bottom line: If you were a Celt who wanted to practice your own religion in Roman-occupied Britain, you could, as long as you didn’t upset the civil service or the military presence. In the same way, Celts and other Britons tolerated the Roman religious presence. One might here be tempted to argue that they didn’t have a choice; they did–they could have made things very difficult for the Romans by burning Roman temples, disrupting Roman religious ceremonies, etc. (Boudicca, in her raid in 61, surely did some of this, since she burned several Roman cities to the ground. Such rash actions were rare, though.)
This tolerance did not, however, extend to the bringers of Christianity to the Island. The new religion was just beginning to hold sway in the Empire when the Romans left Britain, so Roman Christianity was not much practiced, except by the Romano-British the Empire’s evacuation left behind. Missionaries came from the Continent to spread their word to Britain; with this word came intolerance and exclusivity. Ancient religions suffered, as did those who practiced them. The people who lived in Britain had far more religious autonomy under Roman rule than under Christian rule. Unlike Rome, Christianity came to stay.