In 400AD, Anglo-Saxon Britain was in a transitional period as the Romans, who had occupied Britain wholly since the end of the second century, began to be recalled to Rome to fight against attacks from the Barbarians. The last Romans left Britain in 410.

Christianity in Britain after the Romans left was substantially weaker than it was in the fourth century, with the lack of strong Christian leaders in the Romans. Thus, Britain contained pockets of Christianity, particularly in areas that had a strong Roman influence, but this waned as the Romans left, although it did not die out completely.

But the British Isles were an attractive prospect for Christian rulers, who desired the political benefits of a Christianised Anglo-Saxon population due to its location and substantial population.

Most of our understanding of the Christianisation of Britain comes from two sources: The Anglo-Saxon Chronicle and the writings of Bede the Venerable.

We know from Bede's account, and the letters of the Pope himself, that Pope Gregory the Great sent St. Augustine and Italian missionaries to Kent in 597 to begin the conversion process of the British Isles. Gregory conceived the plan in c.596, writing in his letters “the nation of the Angles, situated in a corner of the world, has remained up to now in thrall to tree-trunks and stones.” Thus, Gregory believed that the Anglo-Saxons were pagans and desired Christianity. This was a major event in the history of the Anglo-Saxons, as they now began to retain their own identity of Englishness.

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So, to answer your question, the process of Christianisation had began in Roman Britain, but was disrupted by fleeing Romans and the subsequent fall of the Roman Empire. Britain was largely pagan until a re-conversion to Christianity, beginning with Augustine's mission in 597.

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Bibliography

Christopher P. Jones, "Between Pagan and Christian," Harvard University Press. (2014)
NICHOLAS J. HIGHAM and MARTIN J. RYAN, "The Anglo-Saxon World," Yale University Press. (2013)