It is about allegiance.Nope i don't care if you keep sabbath or follow the mosaic law. Nothing wrong with that, its a beautiful thing. But when you decide that it is part of Christian Doctrine or that the bible tells us that we MUST follow the Mosaic Law, I object with 2000 years of history and scholarly research.
Many keep Sunday as the Lord's day, but the Bible points to the seventh day, and not to the first, as the Lord’s day. Jesus said... “The Son of man is Lord also of the Sabbath.” The fourth commandment declares, “The seventh day is the Sabbath of the Lord.” And by the prophet Isaiah the Lord designates it, “My holy day.” [Mark 2:28; Isaiah 58:13.]
It is clear which day is the Lord's.
This is what I believe...
While the observance of the false sabbath in compliance with the law of the state, contrary to the fourth commandment, will be an avowal of allegiance to a power that is in opposition to God, the keeping of the true Sabbath, in obedience to God’s law, is an evidence of loyalty to the Creator. While one class, by accepting the sign of submission to earthly powers, receive the mark of the beast, the other choosing the token of allegiance to divine authority, receive the seal of God.
GC chap 38.