Answer:
4th century .
Step-by-step explanation:
4th century
During the early 300's, Christians were being still pursued their faith but Christianity became the official religion of the Roman Empire as Constantine embraced himself the religion.
With Constantine the Great, the territories under control of the Romans were forced to convert to the faith as well.
Constantine the Great, became the leader of the Church that would closely work with the Roman state.
Since the Roman Empire controlled most of Europe at that time, Europe soon was later known as Christianity, stretching further to the Baltic states in Middle Ages, noting differentiation from the Asian counterparts, (Turkey).
Today, the widespread religion in Europe is Christianity, still.
Increasingly countries in Southeastern Europe with Muslim majorities and inmigration is forecast to turn the tides in favor of Islam.