First Century Christianity NEVER kept the Sabbath. NEVER. Not once do the Apostles tell the Christians they must keep the Sabbath, or any of the other 613 Commandments of Judaism. Sabbath Keeping has never been a part of genuine Christianity. Only quasi-Christian cults emphasize it.
Under Jewish law, circumcision is required before ANY of the Mosaic Laws can be kept. The Council of Jerusalem in AD 50 decided once and for all that the Gentile Christians would not be required to follow the Mosaic Law. Their duties were described in the Book of Acts and are parallel to the Noahide Law.
Seventh Day Adventism has a yearly stampede of 300,000 fleeing their cruel system of cult bondage. EVERY single significant doctrine of SDA has been soundly debunked by their own theologians! Not one of them has held up under scrutiny of their own Bible scholars! It is a cultish house of cards.
Sabbatarians THINK they are keeping the Sabbath and brag loudly about their superiority over other Christians because of it, but in truth they are nowhere nears keeping the Sabbath. What they "keep" is a convenient, Americanized, sanitized, dumbed down, processed cheese-version of the Sabbath that bears absolutely NO resemblance to the real deal Sabbath as kept by Jesus or anyone in the Old Testament. They have no idea of how to keep the Sabbath.
Don't flatter yourself for imagining you have accomplished something you haven't. The only truly honest Sabbath Keepers are the ones who recently began keeping the monthly, yearly and New Moons Sabbaths, all the Jewish feast days and some of them have built the necessary backyard temples to commence animal sacrifices. The honest Sabbath keepers have searched out and found the Rabbis necessary to follow the Levitical food laws. If you are going to follow Jesus's example of Sabbath keeping, then you also have to follow all the other Mosaic Laws outlined above. He did.
In the aftermath of SDA's Sabbath Scholar Dr. Samuelle Bachiochi's demolition of the Adventist culture that had developed around the 7th Day Sabbath over 150 years, honest Sabbatarians have followed his lead and are undertaking the necessary steps to follow ALL of the Jewish laws that are required. Keeping their sanitized process pasteurized Velveeta cheese-version of the 7th Day Sabbath just does not cut it anymore.
Furthermore, they are beginning to see the necessity of rejecting St. Paul as a "False Prophet" and someone who they claim "illegitimately" introduced Greek and Roman thought into Christianity. They are presently eliminating the Pauline Epistles from their Biblical canon for precisely this reason. Notice how advocates of the Sabbatarian cult NEVER cite St. Paul for support for their cruel system of cult bondage. It is obvious they must discredit St. Paul if they are going to keep all of the Jewish Laws, which the honest among them now do teach and attempt to practice.
But that has nothing to do with Christianity. Sabbatarians have simply started their own religion that is neither Christianity nor Judaism. It is actually a revival of the old Ebionate cult which has long been viewed as a destructive heresy by Christianity. It is in a spiritual No Man's Land. They worship the Jewish Sabbath rather than the Resurrected Son of God.
Christianity has NEVER required Sabbath keeping.