Short answers, pulled from the story.
The Edict of Thessalonica was issued on the 27th day of February in the year 380. This decree made Nicene Christianity the official state religion under the rule of Theodosius I, Gratian, and Valentinian II.
Christian communities existed as minority groups within the empire and faced execution or imprisonment during the Diocletianic Persecution from 303 until 313. Before this period they were not recognized by imperial law and had to refuse participation in the imperial cult.
Three emperors issued the single decree that altered the course of history. These rulers included Theodosius I who governed the eastern half of the Roman Empire alongside Gratian and Valentinian II who governed the west.
The Council at Nicaea previously defined the orthodoxy that Emperor Theodosius I later enforced through imperial law. This document reserved the title Catholic Christians for those following the faith defined by Pope Damasus I of Rome and Pope Peter of Alexandria.
The Byzantine Empire collapsed when Ottoman Turks captured Constantinople in the year 1453. This event occurred centuries after the initial declaration of Christianity as the state religion under Theodosius I.