As the writer of Ecclesiastes stated, “There is nothing new under sun.” Consequently, all Orthodox practices and beliefs have their origins in the Law and Prophets. Judaic traditions evolved into Christian traditions primarily under the early influence of the Apostles.
The first certain written historical evidences we have of the use of the sign of the cross is from the ecclesiastical writer, Tertullian, (230 AD) who tells us that candidates for baptism are marked with a sign of the cross on their foreheads during their time of instruction.
Additionally, Tertullian taught: "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".
St. Cyril of Jerusalem (386 AD) 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"
The Lord in preparing the way for the Messiah, gave to Israel instructions regarding how they were to live out their faith as a 'people set apart', in a practical way. From the earliest Old Testament times, Jews had the practice of wearing certain Scripture texts in a little box on their arms and on their foreheads. This practice had it roots from several passages in the Law, in particular Deut. 6:4-8:
"Hear, O Israel! The LORD our God is one. Therefore, you shall love the LORD, your God, with all your heart, and with all your soul, and with all your strength. Take to heart these words which I enjoin on you today. Drill them into your children. Speak of them at home and abroad, whether you are busy or at rest. Bind them at your wrist as a sign and let them be as a pendant on your forehead."
In this passage the first verse (the "Shema Israel") is tantamount to Judaism's Creed - the Lord is one. This basic truth of one God who is Lord of Israel set the People of the Old Covenant apart from the unbelieving "nations" or Gentiles.
Later in history God will say, "Pass through the city (Jerusalem) and put a mark on the foreheads of those who moan and groan over all the abominations that are practiced within it" (Ezekiel 9:4). Here Ezekiel is speaking of the remnant of the people of God who remain faithful, to set them apart from the unfaithful. The Hebrew word for mark is the same as for the last letter of the alphabet, Tav, which in hand-written form is cross-shaped. It was used in the ancient world to brand animals. In Greek this is Tau, T, the shape of Jesus' actual cross.
In the book of Revelation the servants of the Antichrist are to have their own peculiar mark (Rev. 14:9) to indicate their belonging to him.
So, as Jews wore their sign of belief in God, as the supporters of the Antichrist will wear his mark, so the Orthodox have since antiquity worn and practiced the sign of the cross, as the outward manifestation and proclamation to the world that we are followers of Christ.
The Sign of the Cross expresses belief in the Christian truths of the Holy Trinity and Christ crucified, as well as an expression to the visible and invisible world of our commitment to love the Lord our God with all our heart, mind and strength. Additionally, St. Paul says in 1 Cor. 1:23-24, "We proclaim Christ crucified, a stumbling block to Jews and foolishness to Gentiles, but to those who are called, Jews and Greeks alike, Christ the power of God and the wisdom of God."
Consequently, the sign of the cross is an outward proclamation confirming to God, the saints, the angels, the demons and the world, that we are Christians who are trusting in Christ and calling on Christ, to guide us, help us, protect us and save us.