A term applied to various manual acts, liturgical or devotional in character, which have this at least in common: that by the gesture of tracing two lines intersecting at right angles they indicate symbolically the figure of Christ's cross.
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, such as Catholics are taught to make upon themselves when they begin their prayers, and such also as the priest makes at the foot of the altar when he commences Mass with the words: "In nomine Patris et Filii et Spiritus Sancti". (At the beginning of Mass the celebrant makes the sign of the cross by placing his left hand extended under his breast; then raising his right to his forehead, which he touches with the extremities of his fingers, he says: In nomine Patris; then, touching his breast with the same hand, he says: et Filii; touching his left and right shoulders, he says; et Spiritus Sancti; and as he joins his hands again adds: Amen.) The same sign recurs frequently during Mass, e.g. at the words "Adjutorium nostrum in nomine Domini", at the "Indulgentiam" after the Confiteor, etc., as also in the Divine Office, for example at the invocation "Deus in adjutorium nostrum intende", at the beginning of the "Magnificat", the "Benedictus", the "Nunc Dimittis", and on many other occasions.
Another kind of sign of the cross is that made in the air by bishops, priests, and others in blessing persons or material objects. This cross recurs also many times in the liturgy of the Mass and in nearly all the ritual offices connected with the sacraments and sacramentals.
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. It was prescribed in some early uses that the priest ascending to the altar before the Introit should first mark a cross upon the altar-cloth and then should kiss the cross so traced. Moreover it would seem that the custom, prevalent in Spain and some other countries, according to which a man, after making the sign of the cross in the ordinary way, apparently kisses his thumb, has a similar origin. The thumb laid across the forefinger forms an image of the cross to which the lips are devoutly pressed.
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, in putting of our shoes, at the bath, at the table, in lighting our candles, in lying down, in sitting down, 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; over the bread we eat and the cups we drink, in our comings and in goings; before our sleep, when we lie down and when we awake; when we are travelling, and when we are at rest".
The course of development seems to have been the following. The cross was originally traced by Christians with the thumb or finger on their own foreheads. This practice is attested by numberless allusions in Patristic literature, and it was clearly associated in idea with certain references in Scripture, notably Ezekiel 9:4 (of the mark of the letter Tau); Exodus 17:9-14; and especially Apocalypse 7:3, 9:4 and 14:1. Hardly less early in date is the custom of marking a cross on objects already Tertullian speaks of the Christian woman "signing" her bed (cum lectulum tuum signas, "Ad uxor.", ii, 5) before retiring to rest—and we soon hear also of the sign of the cross being traced on the lips (Jerome, "Epitaph. Paulæ") and on the heart (Prudentius, "Cathem.", vi, 129). Not unnaturally if the object were more remote, the cross which was directed towards it had to be made in the air. Thus Epiphanius tells us (Adv. Hær., xxx, 12) of a certain holy man Josephus, who imparted to a vessel of water the power of overthrowing magical incantations by "making over the vessel with his finger the seal of the cross" pronouncing the while a form of prayer. Again half a century later Sozomen, the church historian (VII, xxvi), describes how Bishop Donatus when attacked by a dragon "made the sign of the cross with his finger in the air and spat upon the monster". All this obviously leads up to the suggestion of a larger cross made over the whole body, and perhaps the earliest example which can be quoted comes to us from a