If you review the Hebrew for Ex. 20:8, you will note that it uses the definite article for the Shabbat. Go ahead and do a search on where that root is used before that point in the Bible as to when there might have been a "THE Shabbat" to remember as a singular event. My point in telling you this is that although the Sabbath has been observed, it might be that the point of remembering THE Sabbath Day has been missed since Joshua crossed the Jordan, along with a whole lot of other important teachings.
Believe it or not, but much of the teaching in the Sermon on the Mount was about the Sabbath year, which was appropriate given that Yeshua's first year of ministry was a Shemitta, which is why the multitudes were available. It is also why the Son of Man had no home, as commanded in Ex 23:11 (you'll need a Hebrew lexicon to understand that point as virtually every historic English translation is a long way from the complete truth, although the Chabad have for the most part corrected theirs since I wrote this).
Yeah, I'm one of those guys the author of the article above would call an apostate. That's why I cited Matthew 7:21-23 above.
I don’t think Seventh Day Adventists are apostate. They acknowledge the Trinity and believe Jesus is God. Their views on eschatology and soul sleep are, in my opinion, a bit heterodox but certainly not vital to saving faith.