 Constantine had returned to Nicomedia from the eastern front by the spring of 303 AD , in time to witness the beginnings of Diocletian 's " Great Persecution " , the most severe persecution of Christians in Roman history . In late 302 , Diocletian and Galerius sent a messenger to the oracle of Apollo at Didyma with an inquiry about Christians . Constantine could recall his presence at the palace when the messenger returned , when Diocletian accepted his court 's demands for universal persecution . On 23 February 303 AD , Diocletian ordered the destruction of Nicomedia 's new church , condemned its scriptures to the flames , and had its treasures seized . In the months that followed , churches and scriptures were destroyed , Christians were deprived of official ranks , and priests were imprisoned . 
