Before the Romans, Angles, and Saxons arrived in Britain, there were many different groups of people. They were first properly mapped by Julius Caeser in 55AD (this map demonstrates how many groups there were at the time). The people did not have one definitive identity, though today we tend to refer to them as 'Celts'. This is due to the language similarity with Gaul (roughly modern-day France) rather than ethnicity [1]. Religious beliefs were pagan and worshipped both 'supreme' gods and smaller, local deities [3].

From 55AD, the Romans invaded and gradually took over southern and eastern Britain[2]. They built town, roads, villas, and forts and by 300AD, 'Britannia' was legally and culturally Roman [1]. Graeco-Roman gods were worshipped in temples which were built in Britain and people were encouraged to worship the Roman Gods though the local Celtic gods were still worshipped [3]. Local gods were merged with the Roman deities and there was no clash between belief systems. In Bath in the West of England, for example, the Roman goddess Minerva was merged with Sul, a local god.

Christianity was first introduced to Britain by the Romans and was popular in the 4th century. There is evidence that Christianity arrived well before then through trade, though there does not appear to have been an attempt to convert the populous [4]. However, pagan traditions were still strong, both the Roman pantheon and the local Celtic traditions [3]. In 407AD the Romans left Britain, with Christianity being a minority belief.

Shortly after, around 450AD, groups of Angles (Denmark) and Saxons (Germany) arrived in Britain along with the Jutes [2]. The invaders were pagans [5] and Christianity disappeared from areas in which they settled, but survived in areas such as Wales and Ireland [4].

Pope Gregory sent missionaries to Britain in 597, converting the Kentish King Æthelberht. With the help of Irish missionaries, the Anglo-Saxons were converted so that by the time the Danes arrived in 851AD, Christianity was the primary religion on the Island [5].

[1] People of Britain, Simon James, BBC http://www.bbc.co.uk/history/ancient/british_prehistory/peoples_01.shtml#three

[2] Settlers: Genetics, geography and the people of Britain, Museum of Natural History, University of Oxford http://www.oum.ox.ac.uk/settlers/

[3] Romans: Religion, English Heritage https://www.english-heritage.org.uk/learn/story-of-england/romans/religion/

[4] Constantine and Augustine, Christianity in Britain, BBC https://www.bbc.co.uk/religion/religions/christianity/history/uk_1.shtml

[5] Religion in the Anglo-Saxon Kingdoms, Alison Hudson, British Library https://www.bl.uk/anglo-saxons/articles/religion-in-anglo-saxon-kingdoms