In 312 the Emperor Constantine (280-337) became the first Roman emperor to convert to Christianity. This brought to an end nearly three hundred years of the persecution of Christians and in 313 he promulgated the Edict of Milan which admitted Christianity as among the religions authorised within the Roman empire. It was shortly to become the official religion of the State. The Emperor Julian (361-3) launched a pagan reaction, but by the end of the century Christianity was firmly established and the Emperor Theodosius brought in sanctions against pagan worship.
Editors. "Christanisation of the Roman empire". The Literary Encyclopedia. First published 16 November 2005
[http://www.litencyc.com/php/stopics.php?rec=true&UID=1627, accessed 21 January 2017.]