What day did God rest after Creation? What day did Moses keep? What day did Jesus and the disciples keep? Which of Gods commandments starts with the words "Remember" to indicate people would either forget or it would be changed? I doubt Sunday keeping is the Mark but the forced actions to worship on a particular day or way or to a particular deity could very easily be the Mark.
***I doubt Sunday keeping is the Mark ****
I don’t believe this is official SDA doctrine, but some of the independent SDA missionaries used to preach it many years ago.