Whereas my first post exegeted the meaning of the Sabbath for Christians, my second post dealt with the popular misconception that Constantine was responsible for Sunday worship. I now wish to dispel that notion entirely, and my evidence for doing so, comes in the form of those ancient churches which have always existed entirely outside the boundaries of the Roman Empire. The Indian Orthodox Church, the Church of Edessa (which became part of the Assyrian Church of the East and the Syriac Orthodox Church), the Armenian Apostolic (Orthodox) Church, the Assyrian Church of the East, the Georgian Orthodox Church, and the Ethiopian Tewahedo Orthodox Church, were founded outside of Byzantine control.
In the case of the Armenian church, it was founded during the Diocletian Persecutions, before Byzantion had even become Constantinople and Christianity legalized in the Imperium Romanum, and in the case of the Indian, Edessan and Assyrian churches, they date back to the years 33-53 Anno Domini, to the ministry of Saint Thomas the Apostle and his disciples Addai and Mari, who travelled east and converted Aramaic speaking Jews and Gentiles, just as Saint Paul with his disciples like Barnabas and Titus, travelled West and converted Greek-speaking Jews and Gentiles. Saint Thomas was martyred in Kerala* in 53 AD.
These churches, of Armenia, Assyria, Ethiopia, Georgia, and India, always existed outside the formal reach of the Empire, and which also were at no time a part of the Roman Catholic Church or subordinate in any way to the Pope of Rome.
Indeed, the Assyrian Church of the East severed communion with the Roman Catholic Church after Pope St. Celestine sided with St. Cyril the Great, contributing to the latter prevailing over Nestorius at the Council of Ephesus, and at the Council of Chalcedon, Pope Leo X wrote a Christological Tome which, in addition to other events and intrigue, led to a break in communion between the Oriental Orthodox churches, including the Armenian and Ethiopian churches, and also the Syriac Orthodox, and Coptic Orthodox churches, and the Chalcedonian churches (the Roman Catholics and Eastern Orthodox).
From that time forward, the Syriac and Coptic Orthodox churches, or to be more precise, the Oriental Orthodox Churches of Antioch and Alexandria, for initially, there were Greek speaking members, but these churches came to be dominated by the local vernacular languages, were enemies of the Roman Empire and were persecuted, particularly harshly by Justinian. Then, the provinces of Aegyptus, Judea and Syria were conquered by the Ummayid Caliphate, and under the early Muslims, the active persecution stopped, even though Christians were forced to endure certain humiliations as dhimmis, humiliations which those weak in the faith could not deal with, which is why in Egypt there are 70 million Muslims and only 10 million Coptic Christians (and only tens of thousands of Alexandrian Greek Orthodox Christians). Of course, more brutal persecutions would come later, including the suppression of the Coptic language as a vernacular tongue by the Mamluks, who would literally cut out the tongue of anyone overheard speaking it.
Now, one thing all of these churches have in common is they all worship on Sunday. The Armenian Kingdom, which was never a part of the Roman Empire, became an enemy of the Roman Empire for some time after the Chalcedonian schism, and converted to Christianity before Constantine rose to power, and the Armenian Orthodox Church is the only church I am aware of which prohibits celebrating Holy Communion on any day except Sundays and feast days (they also continue to celebrate the Nativity together with the Epiphany, that is to say, the Feast of the Baptism of Christ, on January 6th, or January 18th for those in the Holy Land, where the Julian Calendar is used, being the only ancient church not to adopt the fourth century liturgical development of separating Christmas and Epiphany into two different feasts, by moving the former to December 25th). As should be evident, no one compelled the Armenians to worship on a Sunday; the man who converted them, Gregory the Illuminator, was a Greek Christian, and the woman who converted the Georgians, who also worship on primarily on Sundays (but not exclusively; only the Armenians take it that far), was herself an Armenian, St. Nino.
So St. Gregory the Illuminator, who was not Roman Catholic and converted the Armenians in 300-306 AD (when construction began on the Holy Etchmiadzin Cathedral, after a number of people saw our Lord on that location), eight years before Constantine embraced Christianity, taught them to worship on Sunday.
Further proof comes in the case of the Ethiopian and Indian Orthodox churches; there were many Jews in Kerala until the formation of Israel, the most famous Indian Jew being Vidal Sassoon, and many Indian Orthodox Christians are of Jewish descent (some exclusively so).
Moreover, the Ethiopians were entirely Jewish in religion and of partial Hebrew ethnicity (like most Jews who settled outside of the Levant in the Diaspora) before the conversion of the Imperial House of Solomon to Christianity in the early 4th century, but after that conversion, many Jews remained until the Derg Communist regime which murdered Emperor Haile Selassie began threatening the lives of the Jews (known as the Beta Israel, meaning “House of Israel”); most fled to Israel, and some were evacuated through the Sudan in an extremely dangerous operation chronicled in the excellent film Red Sea Diving Resort. The Ethiopian Christian liturgy is very similar to that of the Beta Israel; even the vestments worn by the deacons are similiar, with crosses replacing the Star of David in the Christian version, and Holy Communion instead of animal sacrifices. And the most important day of worship for the Ethiopians is Sunday, as it is for the Assyrians, the Armenians, the Copts, the Egyptians, the Eritreans, the Georgians, the Indians and the Syriac Orthodox, all churches that either formed and always existed completely outside the borders of the Roman Empire and free from control by the Papacy, or that broke relations with the Roman Catholic Church and became enemies of the Roman Empire, and had no reason to maintain a tradition imposed by Constantine.
What unites all of the ancient Christian churches which still exist, and most of those which do not, is that they celebrate their faith in the resurrection of our Lord and Savior Jesus Christ by worshipping primarily on Sundays. And they always have worshipped on primarily on Sundays, because this practice is integral to the Christian faith in so many ways.