Who made Christianity the official religion of Rome?

Answer 1

theodosius I
Christianity was not made the official language of the Roman Empire because it is not a language.

Mainstream Christianity, Western or Latin Christianity and Eastern or Greek Christianity were made the sole legitimate religions of the empire by the Edict of Thessalonica by co-emperors Gratian, Theodosius I and Valentinian II in 380. The purpose of the edict was to ban the other Christian sects, which were branded as heretic. Their persecution started soon after the issuing of the edict. The main target was Arian Christianity, which was popular around the empire.

Later Latin or Western Christianity came to be called Catholic and Greek or Eastern Christianity came to be called Orthodox.

Answer 2

In 380 co-emperors Gratian, Valentinian II and Theodosius I issued the edict of Thessalonica that stated all citizens should profess the faith of the bishops of Rome and Alexandria (Roman Catholicism and Orthodox Christianity). This was a move against the Arian branch of Christianity which at the time was popular, especially in the eastern areas of the empire, and whose doctrine was different. The edict established the Catholic Church (at the time this included both the Roman Catholic and the Orthodox branches of Christianity, which were also called the Creed of Nicene) as the only legitimate imperial religion and the only one which would receive state support. Soon after this there were persecutions of pagans.