~excerpted from CERC by Fr. William Saunders
The sign of the cross is a beautiful gesture which reminds the faithful of the cross of salvation while invoking the Holy Trinity. Technically, the sign of the cross is a sacramental, a sacred sign instituted by the Church which prepares a person to receive grace and which sanctifies a moment or circumstance. Along this thought, this gesture has been used since the earliest times of the Church to begin and to conclude prayer and the Mass.
The early Church Fathers attested to the use of the sign of the cross. Tertullian (d. ca. 250) described the commonness of the sign of the cross: "In all our travels and movements, in all our coming in and going out, in putting on our shoes, at the bath, at the table, in lighting our candles, in lying down, in sitting down, whatever employment occupies us, we mark our foreheads with the sign of the cross" (De corona, 30).
St. Cyril of Jerusalem (d. 386) in his Catechetical Lectures stated, "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 everything; over the bread we eat and the cups we drink, in our comings and in our goings out; before our sleep, when we lie down and when we awake; when we are traveling, and when we are at rest" (Catecheses, 13). Gradually, the sign of the cross was incorporated in different acts of the Mass, such as the three-fold signing of the forehead, lips, and heart at the reading of the gospel or the blessing and signing of the bread and wine to be offered occurs about the ninth century.
The earliest formalized way of making the sign of the cross appeared about the 400s, during the Monophysite heresy which denied the two natures in the divine person of Christ and thereby the unity of the Holy Trinity. The sign of the cross was made from forehead to chest, and then from right shoulder to left shoulder with the right hand. The thumb, forefinger, and middle fingers were held together to symbolize the Holy Trinity — Father, Son, and Holy Spirit. Moreover, these fingers were held in such a way that they represented the Greek abbreviation I X C (Iesus Christus Soter, Jesus Christ Savior): the straight forefinger representing the I; the middle finger crossed with the thumb, the X; and the bent middle finger, the C. The ring finger and "pinky" finger were bent downward against the palm, and symbolize the unity of the human nature and divine nature, and the human will and divine will in the person of Christ. This practice was universal for the whole Church until about the twelfth century, but continues to be the practice for the Eastern Rites of the Catholic Church and the Orthodox Churches.