The Sign is performed by Christians, especially in the Catholic, Orthodox, Anglican and Lutheran churches, during various liturgical or devotional times to remind the worshipper of the cross. Although Protestants generally omit the ritual, only a few of them these days would protest that it's too 'Catholic'.
Its use can be traced back to the 2nd century, when Christians signed a cross on their foreheads in devotion and to ward off evil spirits. (We must remember that in the old days, belief and superstition were mixed. Today, Christians are guided by reasoning.) It is commonly accepted that St. Ignatius of Antioch established the practice as a formal accompaniment to devotions.
The Sign is usually made by the fingers tracing the shape of the cross by first touching the forehead, then the sternum, and finally from shoulder to shoulder. A Hand-held Cross may be used instead of the fingers.
Another form is made when blessing a person or object. In this case, the Sign is traced in the air, rather than touching the body.
And a third form is by tracing a small cross using the thumb, for example on an infant's forehead during baptism. Marking a Sign on the forehead is pretty ancient and this practice might be where Signing originated (see Tau).
Laying the thumb across the forefinger to form a cross makes a handy (!) cross and is popular in some Catholic countries such as Spain.
In each case, the Sign is made where all arms of the cross are roughly equal length, as in the Greek Cross. In printed copies of liturgical text, this 'plus sign' + (or the Latin Cross if the font character is available) is included at the point where the priest is to make the Sign.
When the Sign is made by touching one's body, the fingers start at the middle of the forehead and then move downward to the sternum. This movement down from the head to the heart symbolises Christ coming down from heaven to earth. The fingers then move to the left shoulder and from there to the right shoulder2. This symbolises Christ carrying us from the left, evil side of life, to righteousness on the right-hand side of God. For this same reason, fingers of the right hand are used.
The number of fingers used varies but when two fingers are employed (forefinger and middle finger, common in Eastern Europe), these signify the two natures (human and divine) of Christ. When the third finger (or thumb) is also used (more common in the West), the three are brought to form a single point symbolising the Trinity being the single divine Godhead.
The size of the cross varies according to the situation. One might consider a small Sign of the Cross is appropriate as an outward display of reverence when entering a church, and a larger cross for a Sacrament.
When making the Sign of the Cross, it is common to repeat the Trinitarian Formula from Matt. 28:19 "In the name of the Father, and of His Son Jesus Christ, and of the Holy Spirit. Amen" (or in Latin, "In nomine Patris et Filii et Spiritus Sancti, Amen"). The timing is usually:
- fingers at the forehead: "In the name of the Father,"
- fingers at the sternum: "and of the Son,"
- as fingers move from shoulder to shoulder: "and of the Holy Spirit. Amen."
These words are declaring the presence, and coming into the presence, of God.
There are several other phrases that some people prefer, and some churches are quite dogmatic. But of course, most important are the words said in your heart rather than merely following church tradition. Similarly, whilst there are 'set' times and places where Church etiquette dictates the Sign should be made, the most important times are when you feel it is necessary.