The Church of God claims that Sabbath worship was changed to Sunday worship by the Roman Emperor Constantine in AD321, but this is a distortion of the facts. The Bible is divided into the Old and New Testaments, and the Sabbath in the Old Testament was a day of rest from labor. However, Jesus worked on the Sabbath, healing the sick, and was criticized by the Jews for doing so. Jesus did not keep the Sabbath, saying that because the Father works, I also work. 

The members of the early church thought the world was ending in their day, so they worked without the concept of a day off to preach the gospel. They thought they would be raised on Sunday (the first day of the week) when Jesus rose from the dead, so they considered Sunday to be an important day.

[1 John 2:18] Dear children, this is the last hour; and as you have heard that the antichrist is coming, even now many antichrists have come. This is how we know it is the last hour.

Members of the early church practiced Jesus' words to remember me until I come again by celebrating the Lord's Supper with Jesus' resurrection. To commemorate Jesus' death, members of the early church fasted until 3 p.m. on January 14, the day Jesus died, and then ate the Lord's Supper.

[1 Corinthians 11:26] For whenever you eat this bread and drink this cup, you proclaim the Lord's death until he comes.

Thus, the early church never celebrated anything other than the resurrection of Jesus and the Lord's Supper. Later, the early church flourished and two central churches were established, one in the East and one in the West, but they disagreed over the date of the Lord's Supper. In church history, the Eastern church fasted until 3 p.m. on January 14 of the Jewish calendar, the day Jesus died, and then celebrated the Lord's Supper afterward, while the Western church fasted from 3 p.m. on the 14th until dawn on Sunday, the day Jesus rose from the dead. This dispute lasted for hundreds of years, until in AD325 it was decreed by Constantine that the Lord's Supper be held on Sunday.

The Roman emperor Constantine's involvement in church affairs stems from his own Christianity. A general on the fringes at the time, Constantine went to war to help the king of the Western Roman Empire, and the night before the battle, he had a dream revelation that told him to make the sign of the X, which stands for Christ in Greek. shields of his soldiers and set out on a campaign, which is said to have resulted in a great victory, and from then on he became a devotee of Christianity. Constantine later became king of the Eastern Roman Empire and led the Edict of Milan, which recognized Christianity. Around AD300, the church was brutally suppressed by the emperors, both East and West, because it refused to join the army. This led to a nationwide crackdown to eliminate the church. Emperors burned churches, banned worship, forced people to renounce their faith, and executed those who refused. The persecution was at its worst between 300 and 305, when it is said that each home in Rome was turned into a mourner's house. But because churchgoers made up 10% of the Roman population, when this became a problem and public opinion turned anti-state, the emperors gradually stopped suppressing Christianity. Then, in 306 AD, Constantine became emperor of the Eastern Roman Empire and stopped suppressing Christianity, and in 313 AD, he issued the Edict of Milan, which recognized Christianity as the official religion, returned confiscated property, and proclaimed religious freedom. Sixty years later, in 380, he recognized Christianity as the official state religion and made all citizens Christian.

So the mandate for Sunday worship in AD321 was actually a consideration for the church. At the time, Roman believers held Sunday in high regard because they thought Jesus had risen on Sunday, and they celebrated the Lord's Supper on Sunday. And they thought that on the Lord's Day that Jesus was resurrected, they would also be resurrected, and they had a preliminary gathering to prepare for that day. The Wednesday and Friday services originated from this, and they are services that prepare for the Lord's Day (resurrection).

In response, Constantine declared Sunday closed so that churchgoers could meet freely, but since he did not recognize the church as the official state religion, he proclaimed the first day of the week as the day of the sun in an effort to accommodate various religions. At the time, the majority of Roman soldiers were Mithraists, who worshiped the sun god. Mithras was the largest religion in Rome, so in order to win over the soldiers, they decided to make Sunday the day of the sun, which symbolized Mithras. If the Sabbath was such a central life-and-death truth, as the Church of God people claimed, why were they silent about the Sunday holiday? They had no fear of death, having chosen death over submission to the Roman emperor 15 years before the Sunday holiday was decreed. So the claim that they changed the Sabbath to Sunday worship in 321 is false.