Answer: The practice of tracing the sign of the cross is most prominent in the Roman Catholic church but is also practiced among the Eastern Orthodox and Episcopalians. The history of the sign of the cross goes back as far as Tertullian, the early church father who lived between 160 and 220 A.D. Tertullian wrote, "In all our travels and movements, 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."
Originally, a small cross was traced by the thumb or finger on one's own forehead. While it is difficult to pinpoint exactly when the switch was made from tracing a small cross on the forehead to the modern practice of tracing a larger cross from forehead to chest and shoulder to shoulder, we do know that the switch had occurred by the eleventh century, A.D. when the Prayer Book of King Henry provides an instruction to "mark with the holy cross the four sides of the body."
Catholics find support for the sign of the cross primarily in their many years of church tradition and, secondarily in Exodus 17:9-14 and Revelation 7:3; 9:4; 14:1. While the passages do speak of a sign on the forehead for protection from God's judgment, they must be interpreted in light of their context. On the basis of their context, there is not reason to believe any of the verses prescribe the ritualistic sign of the cross.
In the sixteenth century, one of the central tenets of the Protestant Reformation was sola scriptura, whereby any practice that didn't line up with Scripture was jettisoned. The English Reformers believed the use of the sign of the cross should be left up to the individual, as was written in the Prayer Book of King Edward VI. "...kneeling, crossing, holding up of hands, knocking upon the breast, and other gestures, they may be used, or left, as every man's devotion serveth without blame." Protestants generally viewed the sign as a tradition that was unsupported by Scripture, or even as idolatrous, and it was therefore abandoned by most.
While the Bible does not instruct us to cross ourselves, the sign of the cross is not without biblical symbolism. The shape of the sign is a reminder of the cross of Christ. Historically, the sign has also been viewed as representing the trinity: Father, Son, and Holy Spirit. Through faith in the Lord Jesus Christ and His substitutionary death on the cross, salvation is extended as a free gift to all of humanity. The trinity is the doctrine of the Godhead: one God existing in three distinct persons. Both doctrines are foundational to both Catholics and Protestants and are certainly well-founded biblically. The sign of the cross has at certain points been associated with supernatural powers such as repelling evil, demons, etc. This mystical aspect of the sign of the cross is completely false and cannot be supported biblically in any way.
The mystical aspect aside, tracing the sign of the cross is neither right nor wrong and can be positive if it serves to remind a person of the cross of Christ and/or the trinity. Unfortunately, such is not always the case and many people simply go through the motions of the ritual of signing themselves without a knowledge of why they do it. A final analysis of the sign of the cross is that it is by no means required of Christians because it is not instructed by the Word of God.