Christianity became the official religion of Rome during the reign of Emperor Theodosis I, who ruled from 379 to 395 A.D., according to National Geographic. Theodosis I was the first emperor to decree that all Roman citizens follow Christianity.
Christianity first began to flourish under Roman rule during the time of Emperor Constantine, who converted to the religion in 312 A.D. Although he brought a strong Christian presence to the empire, Roman culture and institutions resisted the change for more than half a century. By the time Emperor Theodosis I ascended to power, the Roman Empire was beginning to crumble. Theodosis I was the last emperor to preside over a united Roman Empire.