Lullingstone Chi Rhos
Imagine Britain before the Romans arrived and before artisans, traders and missionaries brought Christianity to her shores. Since 500 BC various Celtic tribes had lived and fought during the era we call the Iron Age. When they weren't at war with each other, the people were farmers. Traditions were oral, people lived in mud huts in hamlets and children were farmed out to be raised by other families. The power in the land was held by the Druids. The Celts were a tough and barbaric bunch who horrified the Romans (and with what the Romans got up to that took some doing). They loved fighting (and that includes the women), sacrificed humans and liked to cut off the heads of people they had killed in battle and keep them because they thought the power of that person would then pass to them.
Once the Romans arrived at this, their furthest frontier, everything changed. After several attempts, they succeeded in conquering the Celts between about AD43 and AD63. The technological, social, administrative and military changes that the Romans brought revolutionised life in England, or Britannia as they called it. And of course the people of Briton, both the natives and the ones sent by the Romans to settle there, became exposed to the Roman's brand of pagan beliefs.

Roman Army of Julius Caesar Landing in Britain, c.55 A.D
We don't know exactly when the first Christian missionary arrived in England (unless the stories about Joseph of Arimathea are true), but at the same time as the Roman occupation revolutionised Britain there was another quieter and more subtle influence in the form of the Christian message which slowly gained a foothold in these isles.
The Romans aggressively stamped out Druidism, but Christianity still had to compete with the Roman pagan religion and for a long time it looked like it would remain a minority cult (which is certainly how the Romans viewed it to start with).
During the greater part of the Roman occupation, Christians were persecuted by the Romans. Why were the Romans intolerant of Christianity? Christians refused to accept that Roman gods or the Roman Emperor could take precedence over Christ and his teachings. Consequently Christians had to meet and worship in secret. Ironically it was the spread of the pagan Roman Empire throughout so many countries that enabled one religion, Christianity, born in the Mediterranean, to become a universal religion. The Roman infrastructure made it so much easier for the message to spread around the known world.
By the third century a sizable number of people across the whole strata of British society had converted to Christianity. Many were drawn by the idea of a close relationship with a single God rather than the impersonal and uncaring Roman gods. Christian leaders were called Bishops. At first they were chosen by the community, then by the clergy; in other words a hierarchy of sorts was being built up outside of the Roman government. This naturally infuriated the Romans who stepped up the persecution.
The first British Christian martyr was St Alban who converted to Christianity and was subsequently executed in the third century. A third century manuscript explains:
"Alban received a fugitive cleric and put on his garment and his cloak that he was wearing and delivered himself up to be killed instead of the priest.... and was delivered immediately to the evil Caesar Severus."
When told he would be executed for the deception unless he would sacrifice to the pagan gods, Alban declared:
"I worship and adore the true and living God who created all things."
In AD313 Christianity had grown too big for anyone to think it could be squashed any more and the Roman Emperor granted official tolerance for its followers. In AD314 bishops were sent from London, York and Colchester to a Council of the Church in France, suggesting that Christianity now had a substantial following in Britain.
The picture at the top of the post shows the decorated pavement from a villa in Lullingstone from the early fourth century. The mosaic depicts a rather Roman looking Christ , with the two Greek letters behind his head the key to his identity as Chir Rho. This villa has earlier depictions of pagan gods and a later church was added, illustrating the movement of this family over the generations from worshiping the Roman pagan gods to Christianity.
In AD407 the Romans left; the crumbling empire no longer able to sustain a presence here. England entered the Dark Ages.