Answer:
In 313, under emperor Constantine, Christianity became the official religion of Rome.
Step-by-step explanation:
Around 313 Constantine I adopted Christianity as his imperial cult after the Battle of the Milvian Bridge. His legions, which were victorious, fought under the "labarum", a standard with the first two Greek letters of the name of Christ.
That same year, the Edict of Milan legitimized Christianity along with other religions practiced in the Roman Empire. In the year 325, the First Council of Nicaea signaled the consolidation of Christianity under an orthodox edict of Constantine, although this did not cause other Christian groups to come out of illegality. In the year 380, Emperor Theodosius I made Christianity the official religion of the Roman Empire and imposed the edict in 392 that prohibited all pagan cults.