The New Testament indicates that the Jewish Christians held both days holy. Paul evidently preached in the synagogues on the Sabbath, but it was on the first day of the week that the Gentile Christians met to break bread (Acts 20:7). This second sacred day was called the Lord's Day to distinguish it from the Sabbath, and was probably the only one observed by the Gentile converts. There is a hint of their being called to account for observing that day only, in Col. 2:16, where Paul bids them pay no heed to their critics. The Teaching of the Twelve Apostles, written certainly before the year 100 A. D., speaks of the Lord's Day and refers to it as a day of holy meeting and the breaking of bread (chapter 14). The primitive Christians everywhere kept it so solemnly. Pliny, the historian, refers to this fact in his letter to Trajan about A.D. 100. Justin Martyr (A.D. 140) describes the religious worship of the early Christians, their sacramental observances, etc., on the "First Day." Other early writers who make clear and unmistakable reference to the Lord's Day are Dionysius of Corinth, Irenaeus of Lyons (who asserted that the Sabbath was abolished), Clement of Alexandria, Tertullian, Origen, Cyprian, Commodian, Victorious, and lastly Peter of Alexandria (A.D. 300), who says: "We keep the Lord's Day as a day of joy because of him who rose thereon." These evidences cover the first two centuries after our Lord's death and indicate that the Lord's Day is an institution of apostolic sanction and custom. All grounds of doubt are swept away by the fact that Constantine in an edict issued in A.D. 321 honored that day by recognizing it as one sacred to the Christians, and ordered that business should be intermitted thereon. Finally, the Council of Nicsea (A.D. 325) in its official proceedings gave directions concerning the forms of Christian worship on that day, and the Council of Laodicea (A.D. 364) enjoined rest on the Lord's Day. Thus by apostolic) usage, by law and custom, by imperial edict and by the highest councils of the early Christian Church the change has been accepted and approved.
