The Sabbath is the "sign" of God's people, however a individual can be a Sabbath keeping Christian and still have the Mark of the Beast. How so, well by following it in its false practices of worship, which is really worship of the Beast. Lets look at the text closely...
Revelation 14:9-12
King James Version (KJV)
9And the third angel followed them, saying with a loud voice, If any man worship the beast and his image, and receive his mark in his forehead, or in his hand,
10 The same shall drink of the wine of the wrath of God, which is poured out without mixture into the cup of his indignation; and he shall be tormented with fire and brimstone in the presence of the holy angels, and in the presence of the Lamb:
11 And the smoke of their torment ascendeth up for ever and ever: and they have no rest day nor night, who worship the beast and his image, and whosoever receiveth the mark of his name.
12 Here is the patience of the saints: here are they that keep the commandments of God, and the faith of Jesus.
Worshiping on Sunday is a direct way of worshiping the Beast, but there are other ways which we must be careful to avoid. There are pagan rites and rituals and we see the idols it has everywhere, but also pagan days and festivals turned into 'Christian holy days'. We see it in 'Easter'. Think about it.
Revelation 14:9-12
King James Version (KJV)
9And the third angel followed them, saying with a loud voice, If any man worship the beast and his image, and receive his mark in his forehead, or in his hand,
10 The same shall drink of the wine of the wrath of God, which is poured out without mixture into the cup of his indignation; and he shall be tormented with fire and brimstone in the presence of the holy angels, and in the presence of the Lamb:
11 And the smoke of their torment ascendeth up for ever and ever: and they have no rest day nor night, who worship the beast and his image, and whosoever receiveth the mark of his name.
12 Here is the patience of the saints: here are they that keep the commandments of God, and the faith of Jesus.
Worshiping on Sunday is a direct way of worshiping the Beast, but there are other ways which we must be careful to avoid. There are pagan rites and rituals and we see the idols it has everywhere, but also pagan days and festivals turned into 'Christian holy days'. We see it in 'Easter'. Think about it.