Did early Christians keep Saturday or Sunday? While there are various opinions about this, some of those opinions are not based upon biblical or historical fact.

The Bible Sabbath Association, which is not a Church of God group (though it has members that are in the COGs, as well as members who are not), published a series of articles from me on this subject.

The following is in the January-February 2018 edition of its The Sabbath Sentinel magazine which helps give additional historical information:

Sabbath–Post New Testament
By Bob Thiel, Ph.D.
This is the fifth part of a five-part series explaining why certain early documents that are claimed against the seventh-day Sabbath are misunderstood and not actually against it.
In my previous articles, I explained that the Greek word Κυριακήν did not mean Sunday and many have misinterpreted writings from the Didache and Ignatius of Antioch’s Letter to the Magnesians as endorsing Sunday. Thus, the earliest claimed post-New Testament documents cited as “proof” that the seventh-day Sabbath was done away truly did not do so.
Is there other proof?
Yes.
If Jesus and/or His apostles did away with the Sabbath as many claim, then it would be reasonable to conclude that the later faithful disciples would not keep it. Yet, other records and writings demonstrate that the faithful did continue to keep the seventh-day Sabbath.
It is well know that from the first century until about 135 A.D. the first fifteen bishops/pastors of Jerusalem all kept the seventh-day Sabbath and were all circumcised Jews (Eusebius. The History of the Church, Book III, Chapter V, Verses 2,3.& Book IV, Chapter 5, Verses 2-4, pp. 45, 71).
Although they have their own biases, even the historians Philip Schaff and Johann Gieseler correctly noted:
The Jewish Christians, at least in Palestine, conformed as closely as possible to the venerable forms of the cultus of their fathers, which in truth were divinely ordained, and were an expressive type of the Christian worship. So far as we know, they scrupulously observed the Sabbath, the annual Jewish feasts, the hours of daily prayer, and the whole Mosaic ritual (Schaff, Philip, History of the Christian Church, Chapter 9. Oak Harbor, WA: Logos Research Systems, Inc. 1997. This material has been carefully compared, corrected¸ and emended according to the 1910 edition of Charles Scribner’s Sons by The Electronic Bible Society, Dallas, TX, 1998.)
While the Jewish Christians of Palestine retained the entire Mosaic law, and consequently the Jewish festivals, the Gentile Christians observed also the Sabbath and the passover (1 Cor. v. 6-8), with reference to the last scenes of Jesus’ life, but without Jewish superstition (Gal. iv. 10 ; Col. ii. 16) (Gieseler, Johann Karl Ludwig. A text-book of church history, Volume I, Chapter II. New York : Harper & brothers. Date 1857-80).
It is known that early Jewish Christians did keep the Sabbath and biblical holy days. And as prophesied by Isaiah (Isaiah 56:1-7), so did the faithful Gentile Christians (and the Isaiah prophecy still has future application).
Ignatius and Polycarp
The faithful Gentile Christians were also keeping the Sabbath outside of Jerusalem. In the early second century, Pastor/Bishop Ignatius of Antioch wrote:
…to Polycarp, bishop of the Smyrnaeans…So approving am I of your godly mind, which is as it were, grounded upon an unmovable rock, that my praise exceeds all bounds…Do not let those who appear to be trustworthy yet who teach strange doctrines baffle you. Stand firm, like an anvil…Grace will be…always…with Polycarp [Ignatius. Letter to Polycarp. In: Holmes, pp. 194-201].
And according to the later testimonies of Irenaeus, Polycrates, and Tertullian, the leader of the Sabbath-keeping Smyrnaeans [The Martrydom of Polycarp. Verse 8.1. In: Holmes, p. 233], Polycarp, did just that in Asia Minor.
Polycarp also taught that Christians should keep the commandments [Polycarp. Letter to the Philippians. In: Holmes, pp. 206-221].
And on the following sabbath he said; ‘Hear ye my exhortation, beloved children of God. I adjured you when the bishops were present, and now again I exhort you all to walk decorously and worthily in the way of the Lord…Watch ye, and again Be ye ready, Let not your hearts be weighed down, the new commandment concerning love one towards another, His advent suddenly manifest as of rapid lightning, the great judgment by fire, the eternal life, His immortal kingdom. And all things whatsoever being taught of God ye know, when ye search the inspired Scriptures, engrave with the pen of the Holy Spirit on your hearts, that the commandments may abide in you indelible.’ [Life of Polycarp, Chapter 24. (1889) from J. B. Lightfoot, The Apostolic Fathers, vol. 3.2, pp. 488-506]
(Note: While some claim Polycarp also kept Sunday, that appears to be a later edition to the text [Monroy MS. The Church of Smyrna: History and Theology of a Primitive Christian Community. Peter Lang edition, 2015, p. 31].)
According to the The Martyrdom of Polycarp by the Smyrnaeans [The Martyrdom of Polycarp, Bishop of Smyrna, Verses 7.1 & 8.1. Charles H. Hoole’s 1885 translation] and other sources [Sozomen. THE ECCLESIASTICAL HISTORY OF SOZOMEN. Book VII, Chapter XIX.], Christians in Polycarp’s area kept the Sabbath after he died.
And since Polycarp referred to Ignatius as ‘blessed’ and endorsed Ignatius’ letters in his Letter to the Philippians [Polycarp. Letter to the Philippians, verse 13.2. In: Holmes, p. 219], it is logical to conclude that Ignatius was faithful to the same teachings and practices that Polycarp did.
Theophilus
In the late second century, Pastor/Bishop Theophilus of Antioch wrote:
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” (έβδομάς), a name which is adopted by every nation, although they know not the reason of the appellation…God having thus completed the heavens, and the earth, and the sea, and all that are in them, on the sixth day, rested on the seventh day from all His works which He made (Theophilus of Antioch. To Autolycus, Book 2, Chapters XI, XII, XIX. Translated by Marcus Dods, A.M. Excerpted from Ante-Nicene Fathers, Volume 2. Edited by Alexander Roberts & James Donaldson. American Edition, 1885, pp. 99,102).
The early faithful Gentile Christians kept the Sabbath. Until the early third century, there was harmony between the faithful Christians in Antioch and Asia Minor.
Conclusion
Early Jewish and Gentile Christians kept the seventh day Sabbath. The faithful did not believe that the Sabbath was done away.
As explained in early parts of this article, neither the Didache or Ignatius’ Letter to the Magnesians actually mention the term Sunday or directly refer to the first day of the week. The Didache is not an endorsement of early Sunday observance.
Ignatius, himself, would have had to be a supporter of the seventh day Sabbath and would not have written against it. Nor, if properly translated, do Ignatius’ letters ever truly write against the seventh day sabbath–instead they endorse the concept that Christians are to keep the Sabbath in accordance with the commandments and the Lord’s way of living, but not according to the ways endorsed by unbiblical Jewish tradition.
There is no evidence whatsoever that any who professed Christ regularly worshiped on Sunday when Ignatius was alive. The simple fact is that Ignatius’ writings do not support the idea that the early New Testament Church observed Sunday.
The Greek expression he used in Magnesians (κυριακήν ζωντες) simply did not mean Sunday when Ignatius wrote his letters. And there is no other contemporaneous reference that any professing Christian at the time of Ignatius observed Sunday. Available evidence (including the Bible, i.e. Acts 13:42-44; Hebrews 4:9) clearly supports the idea that early Christians kept the seventh day Sabbath.
The first clear reference to Sunday worship, even according to Roman Catholic sources, was from Justin Martyr [Slater T. Sunday. Transcribed by Scott Anthony Hibbs. The Catholic Encyclopedia, Volume XIV. Published 1912. New York: Robert Appleton Company. Nihil Obstat, July 1, 1912. Remy Lafort, S.T.D., Censor. Imprimatur. +John Cardinal Farley, Archbishop of New York]. Yet he held so many heretical positions [e.g. see Justin Martyr. The First Apology and Letter to Trypho], it makes no sense to this author that any could truly consider him to be a true Christian.
On the other hand, Polycarp and those faithful Christians who remained in Asia Minor and Antioch did continue to keep the Sabbath for centuries after Jesus died. Faithful Christians still keep the Sabbath to this day.

Here is a link to the first article I had published in The Sabbath Sentinel: The Sabbath Sentinel: The Didache and the Sabbath.

The published article is basically an extract from my article Another Look at the Didache, Ignatius, and the Sabbath. Because of the restrictions in article length for The Sabbath Sentinel, I submitted just a few pages which they published.

It is my hope and prayer that those who read the submitted articles will see that early church history supports Saturday, and not Sunday, as the Christian day of rest. This article should be able to reach people we have not been able to reach in other ways. It is also my hope and prayer that those who read the articles will see that we in the Continuing Church of God have a true and proper grasp of early church history.

The series of articles I have submitted and expected to be published in The Sabbath Sentinel should also help non-Sabbath keepers realize that the historical evidence points to early, faithful, Christians resting on Saturday and not Sunday.

Interestingly, even to this day, the Eastern Orthodox consider Saturday and Sunday festive days, different from other ones:

In the tradition of our Church, Saturday like Sunday is considered a festal day. Even during the Great Lent the rules of fasting are relaxed on Saturdays and Sundays (Calivas A. The Great and Holy Saturday. Copyright: 2002-2003 Greek Orthodox Archdiocese of America).

And the Roman Catholics realize that the seventh day is the Sabbath:

The sabbath…The sacred text says that “on the seventh day God finished his work which he had done”…and that God “rested on this day and sanctified and blessed it”(Catechism of the Catholic Church, 345. Imprimi Potest + Joseph Cardinal Ratzinger. Doubleday, New York, 1994, p. 100).

Despite these views, both the Eastern Orthodox and Roman Catholics have prophecies against those who keep the Sabbath:

Blessed Hieronymus Agathaghelos (1279): And lo, an evil assembly of the crafty leader, dressed in black mourning apparel…those who were taking in a most hypocritical manner the most holy name of Christ…those were the most filthy citizens of Pentapolis…these are semi-godless men…they will have to pay the price before the public executioner of the Sabbatians. (Tzima Otto, H. The Great Monarch and WWIII in Orthodox, Roman Catholic, and Scriptural Prophecies. Verenika Press, Rock City (SC), 2000, pp. 134,135,240).
Catholic writer and priest P. Huchedé (19th century): “After having weakened the faith of Christ in the minds and hearts of many, he will proceed to show that the law of Moses still prevails; he will re-establish the Sabbath and all the legal observances; and he will invite the Jews to re-establish their nationality, after which he will declare himself the true messiah; he will endeavor to prove the truth of his assertion from Scripture—he will declare his design of rebuilding Jerusalem and the temple and of bringing the whole world under his dominion…According to St. Cyril of Jerusalem (Cath. 15), he will win the esteem and attachment of mankind by his urbane and unbounded kindness…he will work all kinds of miracles…He will therefore appear as performing miracles similar to all those wrought by Jesus Christ…Antichrist being a Jew, will be circumcised; he will observe the Mosaic law…Then by order of the tyrant the continual sacrifice shall be abolished (Dan. 9:27). The holy sacrifice of the Mass shall no longer be offered up publicly on the altars. The Church shall be devastated; the sacred vessels desecrated; the priests shall be scattered…It appears certain, then, that the Roman empire will be completely demolished by Antichrist and that he will substitute another in its place (Huchedé, P. Translated by JBD. History of Antichrist. Imprimatur Edward Charles Fabre, Bishop of Montreal. English edition 1884, Reprint 1976. TAN Books, Rockford (IL), pp. 17,18,26,54).

Persecution of Sabbath keepers has happened throughout history and will happen again (cf. Revelation 12:17; see also the article Persecutions by Church and State).

But, all scholars realize that the first Christians did keep the day we now call Saturday as the Sabbath.

Fourth Commandment: Saturday or Sunday?

Can you keep the Sabbath and your job?

Catholic teachings on the Sabbath, Sunday, and Protestantism

"Beliefs of the Original Catholic Church"

Sermon Series related to the free online book above: