The Sign of the Cross is a hand motion common in the religious devotions and worship services of Roman Catholic, Anglican, Eastern Orthodox, and some Lutheran Christians. It is also become popular for athletes, even those who are not of a denomination that practices it otherwise, to make the Sign after a significant event (e.g. scoring a touchdown).
It is commonly done accompanying the recitation of the words "In the name of the Father, and of the Son, and of the Holy Spirit", a reference to the Trinity. The Sign of the Cross is made by touching the hand to the forehead, the sternum, and then to each shoulder, symbolizing the Cross of Christ (the order in which each shoulder is touched is right-to-left in the Eastern Orthodox tradition, and left-to-right in Western churches).
Earliest history and Biblical underpinning
The making of the cross or the "signing" is understood by the churches that practice it as a "sacramental" rather than a sacrament. That is, it is something done that God uses as a blessing mixed with the faith of the believer, but it is not specifically commanded by Christ, or a specific practice of the Church of the New Testament for the Christians. To understand how it arose and what it means, the source is the Old Testament. This also provides an example of how Biblical concepts and teachings form the matter for apparently unrelated customs.
It is clear that "signs of the covenant (Otot HaBrit in plural - Hebrew) were to be set up in heaven and on earth to be seen by God who faithfully would act (or refrain from acting - the rainbow of the days of Noah that He would see and "remember" and never again send a flood to destroy all life on the earth. See Eucharist for the connection to the Lord's Supper). This idea was carried forth in the Prophet Ezekiel, "...set a sign ("mark" - King James Version) upon the foreheads of the men who sigh and that cry for all the abominations...slay old and young...but come not near any man on whom is the mark..." Ezek. 9:4-6. Here the sign is clearly protective. The Hebrew for "sign" here is the last letter of the Hebrew alphabet, "tav" (Even in Modern Hebrew, tav in all its forms has something to do with sign - "label", "tag", musical note - symbol of a sound, etc.). The way it was written in Biblical days was in the form of a "t", a cross, slightly tilted or on the arm and leg of one side (forming an x), just as in the cognate languages of Hebrew of the times - Phoenecian, Canaanite, Moabite, etc. Our letter "t" comes through the cognate of Hebrew - Phoenecian, via Greek. (Modern Hebrew script is different, being in reality the Aramaic script). The sign of protection, therefore, in Ezekiel was the cross.
The book of the Revelation in the New Testament preserves for us the same idea. Just as those deluded by the beast were to be sealed on the forehead, so the believers of the Lord are to be protected by the sign on the forehead. "Hurt not the earth, neither the sea, nor the trees, till we have sealed the servants of our God in their foreheads". Rev. 7:3. What was that sign? We do not have to guess or theorize to find out, as we have the literature of the earliest Aramaic Church, coming from the time of some of the New Testament Literature. This early Jewish Christian community see Aramaic Church, prior to the work of the day, would pray their "psalms" together, and would "sign" themselves, not on their forehead as Christians of the Roman sphere were doing, but they would involve their whole body, stretching out their arms as the cross bar. And they would tell what they meant, and so the sign continued on through the centuries. This is the earliest literary mention of making the sign of the cross:
"I extended my hands and approached my Lord, For the expansion of my hands is His sign, And my extension is the common cross; That was lifted up on the way of the Righteous One." (A History of Christianity in Asia, Vol. l., Samuel Hugh Moffett, Orbis,{Odes Sol.42:1,2,}, 1998, pg.52)