The Ephesus Theory.
An overview of the Sabbath-Sunday issues, concerning the origins of the Lord’s Day and its relationship to the Sabbath.
The early Church, from the beginning, was worshiping on the Sabbath. They did so because they saw themselves not as a new religion, but as the fulfillment of Judaism. This practice went on for about 15 years until it was necessary to establish a separate identity from Judaism. This was due to the Riots in Rome in the 40s AD.
Emperor Claudius expelled the Jews from Rome over a riot by someone named "Chrestus". Among those expelled were Priscilla & Aquilla. They Traveled to Corinth and were expelled from the Synagogue. In keeping with the Words of Jesus "they would be rejected by the Jews and kicked out of the synagogue" for preaching about Him. When Paul showed up in the synagogue to preach Christ he was informed about these others who had preached the same thing, Priscilla & Aquilla, When he inquired about them he was told they were meeting down by the riverside. Paul went and found them and they began to discuss the problem in Rome.
Prycallia & Aquila would travel with Paul to Ephesus, in Ephesus Paul entered a new phase of evangelism, turning to the Gentiles and establishing the first separate Christian Service apart from Synagogue Worship. Using the hall of Tyrannaus and consulting with Priscilla & Aquilla about the events in Rome, Paul invented a uniquely Christian service on the 1st Day of the week, to teach Christ, keep the peace, and establish a separate identity so outsiders could tell the difference between Jew & Christian. Priscilla & Aquilla would take this practice back to Rome and implement the Lord's Day there.
Later, Apollos, whom Priscilla & Aquilla instructed in Ephesus, would eventually join them in Rome, then carry this new model back to Alexandria and implement it there. The apostles did not envision the Lord's as a replacement Sabbath, nor a day of no work. It was a festival, an addition to their synagogue worship, in which they would attend to Hear the Gospel of Matthew read. The Christian Scripture as we know it, had not been written yet so to hear the Word of God they had to attend the synagogue. As recorded in Acts 15, Col 2 & Galatians.
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The theological basis for the Lord's Day was Pentecost, First Fruits & Lamb Selection Day, all happening on the 1st day of the week. In addition to this, there is the post-resurrection appearance of Christ on the first day of the week. This is the Historical basis and the Biblical basis for the Lord's Day.
P.S. It is possible that a meeting of some type was practiced before Ephesus, but it is not clear. My theory is based on the dating of the book of 1 Corinthians, that it is written after 50 AD. If it is before 50 AD, then it is evidence that some type of service was going on before the establishment of the church in Ephesus.