We can assume that the first Christians arrived in Britain during Roman rule. Yet there isn't much known about the spread of Christianity in Roman Britain due to the contradictory nature of the archaeological findings. And once the legions left the island and the Britons were overrun by the Anglo-Saxon invaders in the 5th century, all of the old Roman culture together with all traces of the Christian faith vanished. Now Thor and other Germanic gods were worshipped throughout England.
The real Christianisation of the British Isles began in 431, when the Pope sent Palladius as bishop to the Irish. Poor Palladius, however, is largely forgotten now because a year later more missionaries arrived in Ireland, among them St. Patrick. Yes, the St. Patrick. The guy is said to have singlehandedly converted the Emerald Island to Christianity as well as to have driven the snakes out of Ireland.
(That last point is a big, fat, whooping lie: When St. Patrick arrived in Ireland, there weren't any snakes to begin with. After the last ice age snakes simply weren't fast enough to reach Ireland before the water did and cut it off from GB. That's why there also aren't any moles in good, old Éire. Much to the delight of Irish gardeners, I'm sure!)
In subsequent years Ireland became a centre of Christian religion and learning as more and more monasteries were established. One of them is on Skellig Michael, a remote island off the coast of County Kerry. The monastery was built in 588 and probably housed about 12 monks.
Irish monks eventually spread Christianity to Wales, Scotland and northern England, where they established new religious centres such as Iona or Lindisfarne.
In 596 the Pope sent a very reluctant St. Augustine and other missionaries to Britain in order to convert the Anglo-Saxons in southern England. St. Augustine also founded the ecclesiastical capital of Canterbury and later became the first archbishop. The Anglo-Saxon kings were quite happy to convert to Christianity, mostly because they thought the hierarchical example of the Christian church would support their royal authority. Furthermore, monks were extremely useful as they could read and write.
But now we had a bit of a problem in good, old Britain: The southern English Christianity was based on the beliefs and practices of the Church of Rome--and these quite differed from those of the Celtic Church. For example there were disagreements about the organization of the church and, curiously enough, about the date of Easter. This conflict was eventually solved by the Synod of Whitby in 664.
The synod was called in by King Oswy of Northumbria. He followed the doctrines of the Celtic Church, but he had married a princess from southern England. Hence, he and his wife celebrated Easter on different dates, which, as you can imagine, was a bit awkward. Therefore, Oswy invited representatives of both churches to Whitby Abbey, where the matter was discussed. Oswy finally decided in favour of the Roman date of Easter, and thus in favour of the Church of Rome and their form of worship.
Soon after Mercia followed the example of Northumbria and as the kings of Mercia eventually controlled all of England south of the Humber, the Church of Rome became the established church in Anglo-Saxon England, while the Celtic Church lost its influence.
A rather curious literary testimony from the time of the Christianisation of the Anglo-Saxons is the poem "The Dream of the Rood," which clearly shows that even when people had been converted to Christianity, their beliefs often mirrored their former pagan religion. In addition, it also serves as an example how Christianity made use of the pagan Germanic context: The text of the poem not only only survives in the Vercelli Book of the late 10th century, but also as a shorter, fragmentary version chiseled on the borders of the Ruthwell Cross and written in Anglo-Saxon runes.
"The Dream of the Rood" deals with Christ's crucifixion, his death and resurrection. Yet despite this Christian frame, the poem is unmistakably Germanic in its descriptions and in its account of the crucifixion: Christ is described as a young Germanic hero, who strips himself for battle with his enemies. There are no references to his humiliations, exhaustion and weakness. Gory effects like the speaking, blood-dripping cross are added for further entertainment value (judging from the descriptions in Beowulf, the Anglo-Saxons loved gory battle scenes!). Here's an excerpt (my translation): in this scene the narrator has a dream vision of the cross which starts speaking to him:
"It was very long ago-- I remember it still--
that I was cut down from the edge of the forest,
removed from my root. Strong enemies took me there,
they made me into a spectacle for themselves there,
they ordered me to raise their felons;
men carried me there on their shoulders, until they set me up on a hill:
many enemies fastened me there. I saw the Lord of Mankind
hasten with great zeal, that he wanted to ascend onto me.
There I did not dare, against the word of the Lord,
to turn away or to fall apart when I saw
the corner's of the earth shake. I would have
felled all enemies; but I stood firm.
Then the young hero stripped himself--it was God Almighty!--
strong and brave; he ascended the high gallows,
courageous in the sight of many, when he wanted to redeem mankind.
I trembled when the warrior embraced me; yet I did not dare to bow to the earth
to fall to the regions of the earth, but I had to stand firm.
I was erected a cross; I raised the powerful king,
the Lord of Heavens; I did not dare to bend.
They pierced me with dark nails; on me, wounds are visible,
open malicious wounds; I did not dare to injure any of them.
They mocked us both together; I was all drenched in blood,
poured out from the man's side after he had sent forth his spirit.
On that hill I have experienced many