Who was the Roman emperor that established Christianity as the state religion?

The correct answer is that Theodosius I established Christianity as the state religion of the Roman Empire. His formal decree, the Edict of Thessalonica in 380 AD, proclaimed Nicene Christianity as the official state religion and mandated that all Roman subjects adhere to it. This marked a significant turning point in the history of Christianity and its relationship with the Roman state, solidifying its position at the center of Roman life and governance.

Nero, while infamous for his persecution of Christians, did not promote Christianity but rather viewed it as a threat to his rule. Augustus, the first Roman emperor, focused on establishing the empire and did not involve himself with Christian matters since the religion emerged after his reign. Constantine, although a key figure in the history of Christianity, legalized it with the Edict of Milan in 313 AD and played a significant role in its spread, but it was Theodosius I who definitively made Christianity the state religion. Therefore, Theodosius I's actions were pivotal in intertwining the Roman state with the Christian faith.