Where did Christianity spread to by AD 400?

By the year 400, Christianity was the state religion of the Roman Empire, which spread from Britain across Europe to much of the Near East and North Africa. In theory, all citizens could be expected to be Christians, and those who remained faithful to their ancient beliefs were subject to persecution. In practice, Christianity was still concentrated in the eastern part of the empire, with the distant provinces still strongly influenced by paganism. The ruling classes of Britain were Christian, but much of the population continued to practise Druidism.