As far as claiming worship on Sunday is of pagan origin, here are a few things to consider.
What day did early Christians worship on?
From Acts 20:7 “On the first day of the week when we gathered to break bread,…”
From Revelation 1:10 “I was caught up in spirit on the Lord’s day…”
Note: A form of the word “Sunday” is in languages such as German, Dutch, and English. However languages such as Spanish, Italian, and French use a form of the Latin word “Dominica,” which means “Lord’s Day” (check out where the Commonwealth of Dominica got its name).
There are also many other early Christian writings which support that the Lord’s Day was Sunday, and that was when Christians worshipped.
Why did early Christians worship on “Dominica?”
From John 20:1 “On the first day of the week, Mary of Magdala came to the tomb early in the morning, while it was still dark, and saw the stone removed from the tomb.”
From Luke 24:1-6 "But at daybreak on the first day of the week they took the spices they had prepared and went to the tomb. They found the stone rolled away from the tomb; but when they entered, they did not find the body of the Lord Jesus. While they were puzzling over this, behold, two men in dazzling garments appeared to them. They were terrified and bowed their faces to the ground. They said to them, “Why do you seek the living one among the dead? He is not here, but he has been raised”
What about prophecy in the Old Testament?
From Psalms 118:22-24 “The stone the builders rejected has become the cornerstone. By the LORD has this been done; it is wonderful in our eyes. This is the day the LORD has made; let us rejoice in it and be glad.”
What does all this really mean?
Let’s look at the connection to the seventh day and covenant.
The Hebrew word “Sheva” means seven, and it also means oath or covenant. When God created the universe, He chose to rest on the seventh day. Why? Does God need rest? No, He rested on the seventh day as an oath or covenant to creation. Our God is awesome, and He does not take His promises lightly. He did something He didn’t have to do in order to show the first people His unfailing commitment to them, no matter how sinful mankind might become.
Let’s go back to the beginning: Genesis 1:1-3, “In the beginning, when God created the heavens and the earth, the earth was a formless wasteland, and darkness covered the abyss, while a mighty wind swept over the waters. Then God said,…” (“God said” is repeated eight times in chapter one in bringing about creation).
Now let’s go to the New Testament to see the connection:
John 1:1-5 “In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God. All things came to be through him, and without him nothing came to be. What came to be through him was life, and this life was the light of the human race; the light shines in the darkness, and the darkness has not overcome it.”
Colossians 1:15-17 “He is the image of the invisible God, the firstborn of all creation. For in him were created all things in heaven and on earth, the visible and the invisible, whether thrones or dominions or principalities or powers; all things were created through him and for him. He is before all things, and in him all things hold together.”
2 Corinthians 5:17 "So whoever is in Christ is a new creation: the old things have passed away; behold, new things have come.”
Our Lord Jesus is the very spoken Word of God, through whom all things came to be. In the beginning, when God established the Sabbath on the seventh day (Saturday), God rested in order to covenant Himself to His creation. Because of the Father’s covenant with creation, the Word (through whom all things were created) then became flesh, sacrificed Himself for our sins, and rose again on the third day. Through the power of His resurrection, He has given us the ability to become a new creation in Him. Jesus established a New Covenant between God and man, which does not do away with the old, but fulfills it. We celebrate this new covenant, this day of new creation, on the day of our Lord’s resurrection (Sunday). This is not the day of the sun, but “Dominica,” the day of the Lord, when God established His New Covenant between Himself and man.