Answer: Christianity
Explanation:
Christianity is the main religion in Europe today and this began in the 4th century CE with the Edict of Milan where Emperor Constantine the Great in recognition for the role of the church in his rise to power granted them legal status. In 10 years Rome accepted Christianity as the state religion.
As the Roman Empire disintegrated in the west, Christianity did not. The Eastern Roman empire continued to practice Christianity and the barbarians that conquered the western Roman empire adopted Christianity as well which culminated in every state in Europe adopting it by the 900CE.
