In 7th century BC, Celts started inhabiting British Isles. Those Celtic tribes of the La Tene culture were Gaels, Britones and Belgae. Britones inhabited the biggest part of Great Britain and that's one of the reasons why the island got that name. Around 55 BC, Romans came to the Britain for the first time. One century later, conquered part of the island became Roman province of Britannia. It took them around three and a half centuries to completely romanize the Celts. During the empire of Romans, Celts were introduced to Christianity. Romans built road network and numerous fortresses. One of the most important facilities built by them was Hadrian's Wall. It was built in order to protect from attacks of Picts. .
Romans also built series of forts on eastern coast which protected them from Saxon's naval attacks. Romans left Britain in 410. Later on, the buildings that Romans made were seen as "works of giants"" by Anglo-Saxons. Germanic tribes, the Jutes, Angles and Saxons, supposedly came to Britain at the invitation of the Britons. The Britons needed protection from Picts. It turned out to be a bad idea because it brought immigration of Germanic tribes to the British Isles. Many Britons retreated to the hills of Wales while others went to France, where they formed Brittany. Newly arrived Celtic tribes became dominant inhabitants of the island. Over time, those tribes blended into what we call Anglo-Saxons. They established several kingdoms: Northumbria, Mercia, Wessex, East Anglia, Kent and others. Those kingdoms were at first pagan, worshipping Germanic gods. In 597, Latin-speaking missionaries came to Kent in order to spread Christianity among Germans. After Kent, they travelled to other kingdoms too. Also, missionaries from Ireland, which had been converted to Christianity in 400's, came to Britain. .
Even though not all the kingdoms accepted new religion at first, they allowed priests to travel and build churches.