The Catechism in Questions & Answers

592. When did Christianity become the state religion in the Roman Empire?

After some difficult times of persecution, the Roman Emperor Constantine the Great proclaimed religious freedom for the Christians in the year 313.
In the year 381, Emperor Theodosius elevated Christianity in the Roman Empire to the status of state religion. He forbade the worship of pagan gods.

“Religious freedom” refers to a condition in which people are free to profess and practise the religion and worldview of their choice.