Who made Christianity the official religion of Rome?

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.