The ancient Romans first took Christianity to Britain. We know that there were Christians in Britain by the beginning of the fourth century, and by 429 the ruling class of Britain was entirely Christian.
However, after the fall of the Roman Empire, Britain once more became a largely pagan region. In 597, Pope Gregory sent the monk Augustine to England to evangelise the island. Augustine visited the king of Kent who agreed it would be politically valuable to have continental support, and so became Christian.