It is sometimes wondered when the early church transitioned from worshiping on the Sabbath to worshiping on the first day of the week. The answer is revealed to us from the very beginning of the church.
The New Testament church was established in Acts 2. The church began and gathered for the first time on a Sunday (Pentecost was always on the first day of the week), and Scripture reveals that they “continued steadfastly” in the same doctrines and practices in which they began (Acts 2:42). They did not become Christians and members of the N.T. church in order to continue in the principles and practices of the old system of Judaism. Christ had freed them from the old; all things had become new! From the beginning, it was the first day of the week!
A summary of the New Testament evidence (cf. Psa. 119:160) reveals that the early church was assembling on the first day of the week in order to worship the Lord. The purpose of their “coming together” is plainly revealed in 1 Corinthians 11—it was “to eat the Lord’s Supper” (v. 20). We have an example in Acts 20 of the church coming together for the purpose of eating the Lord’s Supper, and Scripture is quick to tell us that it was “on the first day of the week” (v. 7). The day on which the early church was commanded to assemble was “the first day of the week” (not the Sabbath) (1 Cor. 16:1-2). Gathering all N.T. evidence makes it clear that God had selected the first day as “the Lord’s day” (cf. Rev. 1:10).
But, what about all those passages that show Paul and others going to the synagogue on the Sabbath? Do those passages imply that God was either still requiring Sabbath day worship or at least accepting it as much as Sunday worship? There is not one verse in the New Testament that teaches or suggests that they ever were worshiping on the Sabbath in the synagogue! Paul was given “an open mic” at the synagogues on the Sabbath (cf. Acts 13:14-15), and he took advantage of and used the opportunity to preach Jesus as the Christ (cf. 9:20). He was not present to participate in or condone Jewish traditions (read the book of Galatians), but he was present to preach Christ to a people who should have been the most receptive to such a message. The burden of proof is actually on those who claim Paul was there to worship.
When did the early church stop observing the Sabbath and start worshiping on Sunday? When did they stop offering animal sacrifices? When did they stop preaching “Someone is coming”? When did they stop observing the Passover or the Day of Atonement? When did they stop approaching God through a priest? When the church was established in Acts 2, God’s plan was culminated and His new law was put in force.