The mark of the beast cannot be Sunday observance because the whole world will receive this mark including Buddhists, Muslims and vast multitudes of others who will not observe Sunday.
The mark of the beast is to follow men in rebellion against God and God’s commandments, and that includes Sabbath and High Day polluting, as is common place in the COG Groups today.
That mark, the mark of Satan has been on the earth since Eve disobeyed her Creator; and believe it or not: The vast majority of people in the COG Groups who accept their organization or leaders teachings above the commandments of God: have the mark of the beast today.
As we learned in our studies through the teachings of Christ and the Epistles: To have the mark of God we must keep all of the commandments of God!
Those who compromise and tolerate sin, those who idolize men and organizations: ALL have the mark of the beast!