The Roman Empire

Edict of Thessalonica (380 CE): Emperor Theodosius I made Trinitarian Christianity the official state religion of the Roman Empire by issuing the Edict of Thessalonica in 380 CE, which declared Nicene Christianity (the form

of Christianity defined at the Council of Nicaea) as the

official state religion of the Roman Empire. This effectively outlawed other forms of Christianity

(like Arianism which rejects trinity) and traditional

Roman polytheistic practices. Therefore, both the Western and Eastern Roman Empires were predominantly trinitarian Christian after 380 CE.

Closing of Pagan Temples (391 CE): Emperor Theodosius I enacted strict legislation banning traditional sacrifices and official temple rituals. This initiated a gradual, multi-decade process across the empire that systematically phased out public pagan practices in favor of Nicene Christianity.