In the very early years of its history the Eastern Orthodox Church adopted the custom of using the Paschal sermon of St. John Chrysostom at the Paschal Vigil service held during the Saturday night before Easter morning. Chrysostom first proclaimed this sermon as instructions to catechumens, new Christian converts, who were baptized during that vigil service.
Saint John Chrysostom
St. John Chrysostom (the name means "golden mouthed") was one of the most famous preachers and reformers of the second half of the fourth century. He was a priest in his native city of Antioch, Syria, and later became Patriarch of Constantinople. Beginning in 390, he preached a famous series on the New Testament, including ninety sermons on Matthew, eighty-eight on John, and thirty-two on Romans. His reforms to purify the church brought him banishment; he died at age fifty in the year 407 during a forced march into exile.