Gentile-Christians were worshipping on Sunday before the end of the first century.
I don't doubt that. And every other day of the week. But they were still honouring the Sabbath as well.
Paul had prophesied that apostasy would take place soon after his departure. He said there would be a falling away from the truth. One doesn’t have to read very far in early church history to see just how that prophecy was fulfilled. Gnosticism began to rise up under the influence of philosophers who sought to reconcile Christianity with Paganism. At the same time, a strong anti-Jewish sentiment became more widespread. Very speculative interpretations began to appear regarding some of the great doctrines of Christ and the apostles.
You will notice as a historian yourself how anti Sabbath laws increase of the centuries and penalties became heavier as time went forward. There were Sabbath keepers in every century. There would have been more no doubt had the establishment not interfered with persecution and coercion to honour Sunday.
2nd Century Historical Highlights
Gnostic 8th Day Origins
They affirm that man was formed on the eighth day, for sometimes they will have him to have been made on the sixth day, and sometimes on the eighth, unless, perchance, they mean that his earthly part was formed on the sixth day, but his fleshly part on the eighth, for these two things are distinguished by them.
- Irenaeus of Lyons, c. 2nd Century
Theophilus, Bishop of Antioch (120 - 190 A.D.)
And on the sixth day God finished His works which He made, and rested on the seventh day from all His works which He made. And God blessed the seventh day, and sanctified it; because in it He rested from all His works which God began to create.... Moreover, [they spoke] concerning the seventh day, which all men acknowledge; but the most know not that what among the Hebrews is called the "Sabbath," is translated into Greek the "Seventh" (ebdomas), a name which is adopted by every nation, although they know not the reason of the appellation.
Justinian I Enforces Sabbath Prohibition 2nd century.
If any Samaritan, after having proved himself worthy to receive baptism, should return to his former error and be detected in observing the Sabbath, or in doing anything else which proves that he was only baptized through simulated conversion, We order that he shall be proscribed, and sentenced to exile for life.
- Novellae 144, Novellae Constitvtiones Dn. Jvustiniani Sacratissimi Principis, 1717
Council of Laodicea 4th century.
Christians must not judaize by resting on the Sabbath, but must work on that day, rather honouring the Lord's Day; and, if they can, resting then as Christians. But if any shall be found to be judaizers, let them be anathema from Christ.
The 4th century Apostolic Constitutions
. . . Do make the following constitutions. Let the slaves work five days; but on the Sabbath day and the Lord’s day, let them have leisure to go to church for instruction in Piety. We have said that the sabbath is on account of the creation, and the Lord’s day of the resurrection.
- Apostolicarum Constitutionum & Catholicae Doctrinae Clementis Romani libri 8. Francisco Turriano Societatis Iesu Interprete è Graeco, 1578
Sidonius (5th Century)
It is a fact that it was formerly the custom in the East to keep the Sabbath in the same manner as the Lord's day, and to hold sacred assemblies: wherefore Asterius calls Sabbath and Sunday a beautiful span, and Gregory of Nyssa calls these days brethren, and therefore censures the luxury and the Sabbatarian pleasures; while on the other hand, the people of the West, contending for the Lord's day, have neglected the celebration of the Sabbath, as being peculiar to the Jews.
So also Tertullian in his apology: 'We are only next to those who set apart the day of Saturn for rest and luxury.' It is therefore possible for the Goths to have thought, as the foster-sons of the discipline of the Greeks, that they would keep the Sabbath after the manner of the Greeks." "I would also not refrain from telling about that Sabbatarian luxury, for how is it possible to conceal that in public characters?
No man free born or slave, Goth, Roman, Syrian, Greek, or Jew shall do any kind of work on the Lord's day nor shall they yoke up cattle excepting in case of necessity but if any one should presume to do it the free born shall pay the magistrate six solidi and the slave shall receive one hundred stripes
- Council of Narbonne, Canon 4, 589AD
8th Century Historical Highlights
Attempts to Forge and Coerce
Unless ye observe Sunday saith the Lord within its proper boundaries there shall come great tempests and many fiery lightnings and thunder and sulphurous fire which shall burn tribes and nations and heavy stony hail storms and flying serpents and heathens shall come to you from Me saith God Himself even a race of Pagans who will carry you into bondage from your own lands and will offer you up to their own gods. There are moreover five huge beasts and hideous in the depths of hell seeking to come on earth to men to avenge the transgression of Sunday unless God's mercy should hold them back.
-Cain Domnaig c. 8th century, Allegedly found by Conall son of Ceolnian on the altar of St Peter and claimed to have been written by Christ Himself