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so how did they know if one had this mark? as it is not visible?
This questions takes us to the mark on the right hand.
According to catholic literature:
Most commonly and properly the words "sign of the cross" are used of the large cross traced from forehead to breast and from shoulder to shoulder...
A third variety is represented by the little cross, generally made with the thumb, which the
priest or
deacon traces for example upon the book of the Gospels and then upon his own forehead, lips, and breast at Mass, as also that made upon the lips in the "Domine labia mea aperies" of the Office, or again upon the forehead of the infant in Baptism, and upon the various organs of sense in Extreme Unction, etc.
Still another variant of the same holy sign may be recognized in the direction of the "Lay Folks Mass Book" (thirteenth century) that the people at the end of the Gospel should trace a cross upon the bench or wall or a book and then
kiss it...
Of all the above methods of venerating this life-giving symbol and adopting it as an emblem, the marking of a little cross seems to be the most ancient. We have positive evidence in the early Fathers that such a practice was familiar to
Christians in the second century.
In all our travels and movements", says
Tertullian (De cor. Mil., iii),
in all our coming in and going out, we mark our foreheads with the sign of the cross
in putting of our shoes, we mark our foreheads with the sign of the cross
at the bath, we mark our foreheads with the sign of the cross
at the table, we mark our foreheads with the sign of the cross
in lighting our candles, we mark our foreheads with the sign of the cross
in lying down, we mark our foreheads with the sign of the cross
in sitting down, we mark our foreheads with the sign of the cross
whatever employment occupieth us, we mark our foreheads with the sign of the cross".
On the other hand this must soon have passed into a gesture of benediction, as many quotations from the Fathers in the fourth century would show. Thus
St. Cyril of Jerusalem in his "Catecheses" (xiii, 36) remarks:
let us then not be ashamed to confess the Crucified. Be the cross our seal, made with boldness by our fingers on our brow and in every thing; we mark our foreheads with the sign of the cross
over the bread we eat we mark our foreheads with the sign of the cross
the cups we drink, we mark our foreheads with the sign of the cross
in our comings and in goings; we mark our foreheads with the sign of the cross
before our sleep, when we lie down and when we awake; we mark our foreheads with the sign of the cross
when we are travelling, and when we are at rest". we mark our foreheads with the sign of the cross
...so with only your right hand you made the sign of the cross hundreds of times a day!
CATHOLIC ENCYCLOPEDIA: Sign of the Cross