Genesis 4:15 tells of God "marking" Cain, and while no one knows whether or not the "mark" was on Cain's forehead or hand, we do know men would recognize it as God's mark, and know they were not to harm him.
In Ezekiel 9:4, we read of God's protecting "mark" was to be placed on those who were grieved over the sin that was occurring in Jerusalem.
Another word for "marked" is "sealed." The Word says that Jesus was "sealed" (Jn. 6:27); and born again believers are sealed (2 Cor. 1:22; Eph. 1:13; 4:30). Since we are "sealed" until the day of our redemption, "sealing" "marks" us as God's property!
And in Revelation 9:4-5, the "mark" was on all of the foreheads of those who belonged to God, and they could not be harmed by the "locusts." So, by the time the "second Beast" begins "marking" those who worshipped the "first Beast's image," all of God's children had been "marked."
Notice what Revelation 7:3 says concerning the 144,000 of the tribes of Israel: "Saying, Hurt not the earth, neither the sea, nor the trees, till we have sealed the servants of our God in their foreheads." By the time Revelation 13 comes along, with the Satan-possessed Son of Perdition, and the False Prophet with his "mark," I believe that all of God's children will already have been "marked" by God! The Deceiver will not be able to "mark" those whom he cannot deceive (Mt. 24:24; Mk. 13:22).
Every man will be sealed; some by God, but unfortunately, most by the Antichrist!