History shows that there were three fairly brief periods of official, widespread persecution (249-251, 257 and 303-311), apart from which most Roman emperors were tolerant of Christianity. The last and most important period is called the Great Persecution.
Galerius issued the Edict of Toleration, permitting freedom of religion throughout the Roman Empire and thereby ending the Great Persecution of Diocletian. Constantine subsequently extended privileges to the Christian Church by proclaiming the Edict of Milan.