The Mark is not a tattoo. It's not a chip. To have a mark on your forehead, or third eye, is to set your thoughts on the service of the World's Kingdom. To have a Mark on your hand is to put your works toward that Worldly Kingdom. The Mark of the Beast is about serving and working for the AntiChrist who will appear as Christ. Similar to how the AntiPope appears as the Pope.
He will be so good at pretending to be God that most will fall for it. And the first thing he does is bring Justice which God seems to have abandoned (his accusation).
For reference there are other Marks. The mark put upon Cain and the mark of Solomon. Interestingly, the mark of Solomon is a six pointed star. The Star of David. A star with 6 points, six sides of a cube and 6 triangles. The number 666 from the Mark of the Beast is the service of Israel which the World will do in the end.