***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.
I have no idea as I broke from them many years ago. My reading of the Bible is the Mark will be an enforced sign of obedience. The hand or forehead would be symbolic of action (hand) and the forehead would be symbolic of how you think. In other words people will be forced to change how they think and what they do in a way contrary to Gods ways. You see evidence of that now with television. We are being told to be "tolerant" of homosexuality (Real meaning is acceptance). The anti "Stand your Ground" crowd and "guns are evil" that are coming out now are trying to change how people feel about self defense of their home and family. The division of mans law and Gods Law should always be examined to see if Caesar is claiming that which is Gods alone to direct.