The Sign of the Cross a simple form of Christian devotion, used by Catholic and Orthodox Christians, is an outward sign (a sacramental) of our belief in Christ and of our faith in the redemption which flows from His Cross. When we make this sign we also invoke the Blessed Trinity, 'In the name of the Father, and of the Son, and of the Holy Spirit, Amen.
When we make this sign we affirm our faith in Christ crucified and ask for His blessing and protection. It is also a gesture of reverence to the Blessed Sacrament.
Making the sign of the cross is probably derived from Ezekiel's prophetic vision of judgment, in which the Lord commands that a 'mark be set upon the foreheads' of the Israelites who cry out against the evil which surrounds them, so that by this mark God's people were identified as belonging to Him and saved from annihilation (Ezekiel: 9:4-6). Other biblical references to 'sealing' God's people with a sign on their heads are found in the Apocalypse (Revelations) 7:4, 9:4
The sign of the Cross was in general by the second century. Tertullian recounts that 'in all our travels in all our coming in and going out, in putting on our shoes, at the bath, at the table, we mark our foreheads with the sign of the cross.' This sacramental 'mark' is important to Catholics to this day. We are anointed, at baptism and at confirmation, by the priest making the sign of the cross on our foreheads with the Oil of Chrism (the oil blessed by bishops at the Mass of Chrism on Holy Thursday). The sign and the chrism are is also used at the ordination of a priest or bishop. In administering the sacrament of the sick the priest anoints the person with the sign of the cross made with blessed oil. Also, on Ash Wednesday, our foreheads are marked by the priest with the sign of the cross made with blessed palm ashes.
There are two forms of the Sign of the Cross:
The Great Sign of the Cross: (This is the one most people think of, and the one people use most often.) A cross is traced with the right hand, touching the forehead, the chest, then the left and right shoulder. [In Orthodox churches, from right to left.] The Doxology is said aloud or silently as the sign is made.
The Little Sign of the Cross: A cross is made on the forehead with the thumb or index finger (this form is used by the priest when anointing or administering ashes). Or a cross is traced with the thumb on one's own head, lips and heart, a gesture which asks Christ to instruct our minds, aid us in our witness, and renew our hearts. (This sign is made at the reading of the Gospel by both priest and people.)
Another form of the sign of the cross is made by the priest several times during the celebration of Mass and when he grants absolution and gives other priestly blessings, by making an invisible cross with the the first two fingers and thumb of his right hand extended. A similar gesture of blessing is made when a priest blesses religious objects (these objects used in worship are also called sacramentals), such as rosaries, medals, vestments and articles used used in connection with Mass.