Final answer:
Christianity became the official religion of the Roman Empire in 380 AD when Emperor Theodosius I issued the Edict of Thessalonica.
Step-by-step explanation:
Christianity became the official religion of the Roman Empire in 380 AD. The emperor Theodosius I issued the Edict of Thessalonica, which declared Christianity as the state religion and banned other pagan worship. This marked a significant turning point in the history of Christianity and the Roman Empire.
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