Edict of Milan
The Edict of Milan was an agreement between Western Roman Emperor Constantine I and Emperor Licinius, granting tolerance and freedom of worship to all religions, specifically Christianity. After this decision was announced, Constantine declared himself a Christian. It was only after the Edict that it became possible to build churches and cathedrals.
It is important to note that the Edict did not establish Christianity as the official religion of the Empire — that would only happen 67 years later, with the Edict of Tessalonica.
Constantine I was Roman emperor from 306 to 337 CE. Realizing that the Roman Empire was too large for one man to adequately rule, Emperor Diocletian (284-305 CE) split the empire into two, creating a tetrachy or rule of four. While he ruled the east from Nicomedia as an “augustus” with Galerius as his “caesar,” Maximian and Constantius the Pale ruled the west. It was the son of Constantius, Constantine, who would one day rise to defeat all challengers to the throne and reunite the split empire, moving the capital away from Old Rome and build a new capital, a capital that one day would bear his name, Constantinople.