There was no single dominant religion in the Roman empire. Various faiths co-existed together, including the worship of Greek and Roman gods, as well as pagan beliefs. There were probably various religious temples in Londinium and one was certainly for worshippers of Mithras. Christianity became prominent during the reign of Constantine in the early 4th century.
Roman civilisation reached its zenith during the mid-1st century at around the same time that Londinium was being established. While the town matured and grew on the western extreme of the empire during the 2nd and 3rd centuries it was protected, and occasionally prospered, from troubles in Rome and elsewhere. In 260 the empire was over-run by Alamanni, Franks and Goths from the north, Moors from the south and Persians from the east. Only Rome, the Italian peninsula and Britain remained untouched. The Romans fought back but large areas of the empire were lost forever.
Britain was not entirely immune from attack. The Franks, based in the lower Rhineland areas, began making raids on the wealthy and vulnerable east of Britain and the Thames estuary. In the early 3rd century, about one hundred and fifty years after their arrival, the Romans replaced Londinium’s defensive mound with a wall over three kilometres long, six and a half metres high and enclosing an area of three hundred and twenty six acres.
Londinium went into a long, slow decline during the 4th century. It had become successful partly because it was the most convenient port from which to trade with the Rhineland and near Continent. As the Roman armies lost control of the northern Continental areas and it became safer to ship goods to and from Boulogne the ports on the south coast of Britain became more suitable. By the end of the 4th century Winchester had overtaken Londinium as Britain’s leading commercial centre. The town’s population gradually dwindled, concentrated along the riverside. Many buildings were no longer used and earth was laid over derelict structures so that the land could be used for farming. Even into the Middle Ages half of the land within the city walls was being farmed as fields, orchards and gardens.
Barbarian attacks were becoming more threatening to Britain and the population appealed to Emperor Honorius for help. Rome itself was under immediate threat however from the Visigoths who briefly entered the city in 410 carrying away much of its valuable possessions. After the Visigoths had already left Rome Honorius replied to the British that he was unable to help them at that moment and authorised the people to take care of their own defences. Britain was from then on no longer a province of the great empire to which it had been part for three and a half centuries.
With thanks to Ursula Jeffries for help with fact-checking and proof-reading.