Or maybe they are just like me and you and believe that obedience to the commands of God is the expected and obligatory sign of the presence of saving faith. Except that they just think obedience to the literal Sabbath is included in that obedience.
It's not a big deal. Really it's not. They believe in Christ too. Paul says let them think and believe what they want. This is what Paul says to those who insist on setting them straight:
6He who observes the day, observes it for the Lord
3The one who eats is not to regard with contempt the one who does not eat, and the one who does not eat is not to judge the one who eats, for God has accepted him. 4Who are you to judge the servant of another? To his own master he stands or falls; and he will stand, for the Lord is able to make him stand.
They do it for the Lord.
Don't treat them with contempt.
God has accepted them.
Who are you to judge the servant of another?
They are accountable to God, not us, and God is able to make them stand.