A quote from the Constitutions of the Holy Apostles, a document from the late 300s AD, is often provided by Adventists to show that Christians kept the sabbath then:
“Thou shalt observe the Sabbath, on account of Him who ceased from His work of creation, but ceased not from His work of providence: it is a rest for meditation of the law, not for idleness of the hands.” “The Anti-Nicene Fathers,” Vol 7,p. 413. From “Constitutions of the Holy Apostles,” a document of the 3rd and 4th Centuries.
– source on Sabbath Truth (sic) site
What the Adventist teachers won’t easily show you are the following quotes from the same document:
Be not careless of yourselves, neither deprive your Saviour of His own members, neither divide His body nor disperse His members, neither prefer the occasions of this life to the word of God; but assemble yourselves together every day, morning and evening, singing psalms and praying in the Lord’s house: in the morning saying the sixty-second Psalm, and in the evening the hundred and fortieth, but principally on the Sabbath-day. And on the day of our Lord’s resurrection, which is the Lord’s day, meet more diligently, sending praise to God that made the universe by Jesus, and sent Him to us, and condescended to let Him suffer, and raised Him from the dead.
– book II, section LIX
Do you therefore fast, and ask your petitions of God. We enjoin you to fast every fourth day of the week, and every day of the preparation, and the surplusage of your fast bestow upon the needy; every Sabbath-day excepting one, and every Lord’s day, hold your solemn assemblies, and rejoice: for he will be guilty of sin who fasts on the Lord’s day, being the day of the resurrection, or during the time of Pentecost, or, in general, who is sad on a festival day to the Lord. For on them we ought to rejoice, and not to mourn.
– book V, section XX
By taking away context, they hide the truth that these were not Sabbath keepers, but Sunday keepers.
At best, Adventists can argue that these were Sunday keepers who had retained the Sabbath as a custom. Today such practices are ignored and such Christians lumped together as Sunday keepers, while such practices seen in the past are, without evidence, assumed to be cases of Jew-like and Adventist-like Saturday legalism.
However, that’s not what the section of the Constitutions that Adventists quote is telling us. The section they quote is about the 10 commandments, and amongst the 10 commandments is the commandment about keeping the sabbath. The document is not instructing Christians to keep the sabbath. It’s citing the 10 commandments. That’s not honest quoting on the part of Adventists.