The prophet Ezekiel has a vision in which he sees great sins committed by God’s people. But at the urging of a heavenly messenger, the godly men and women who lament the wickedness of their people are marked with an “X” on their foreheads. Bearing that mark, they will be spared the divine judgment that is to come (see Ez 9:1-7).
St. John’s vision in Revelation includes a close parallel to this scenario. Before the angels of judgment are allowed to devastate a wicked world, a seal is placed on the foreheads of “the servants of our God” (see 7:1-3; 9:4). Later, this seal is described as the name of Christ and of his Father (see 14:1).
The corresponding scene in St. John’s vision most likely reflects the Christian baptismal ceremony of his day. This rite included (again, as it still does) the spoken words” in the name of the Father, and of the Son, and of the holy Spirit” (Mt 28:19). The Sign of the Cross on the forehead may also have been part of the rite by that time. As early as the second century, making the Sign of the Cross was a common and well-established custom.