Which decree, issued by Theodosius I, made Nicene Christianity the state religion?

• A. Theodosius I's Edict of Thessalonica
• B. Edict of Milan
• C. Donation of Constantine
• D. Edict of toleration

Answer: (D)

The key idea is how a ruler can establish one faith as the official state religion for the empire. Theodosius I achieved this with the Edict of Thessalonica, issued in 380, which declared that the empire should follow the Nicene (Catholic) faith as the true and official religion and that subjects must conform to it. This move made Nicene Christianity the established church of the Roman Empire, shaping religious policy for generations and suppressing other Christian groups and pagan practices.

By contrast, the Edict of Milan in 313 granted tolerance to Christians without declaring a state faith, the Donation of Constantine is a later forged document with no real bearing on imperial policy, and the term Edict of toleration is too vague to denote a formal establishment of a state church. Thus, the decree that truly established Nicene Christianity as the empire’s official religion is the Edict of Thessalonica.