The religious policy of Constantine I carried out following the edict of Galerius (311 AD) is discussed in the article. The author considers that Constantine I greatly contributed to the elaboration of this decree before the Niceae Ecumenical Council in 325. Based on relevant sources and historical facts, the author comes to a conclusion that the fourteen-year period – 311-325 AD – when Constantine I was involved in religious activities, was the most interesting period in the history of Rome. Christianity, which had earlier been persecuted, became a state religion thanks to the consistent policy of the first Christian Emperor – Constantine I.