Crossing one’s self recalls this seal, and the invocation that is said while making this holy sign calls on our God — the Father, His Son, and the Holy Ghost — and is a sign of our belief; it is both a “mini-creed” that asserts our belief in the Triune God, and a prayer that invokes Him. The Sign of the Cross (often accompanied by spoken or mental recitation of a trinitarian formula) is done by individuals as a prayer and by clergy upon others as an act of blessing. The words “sign of the cross” are used for the large cross traced from forehead to breast and from shoulder to shoulder, such as Catholics are taught to make upon themselves when they begin their prayers. The Sign is required at certain points in the Holy Mass: during the introductory greeting of the service, before the Gospel reading (small Signs on forehead, lips, and heart), and at the final blessing.
The first mention of Christians making the Sign of the Cross is in the 2nd Century. Making the Sign of the Cross before was simply writing a small cross on your forehead using your finger. The sign was used to drive away evil spirits. But today, the sign itself is considered to be a prayer.
According to Pope Benedict XVI (Angelus, September 11, 2005) “making the sign of the cross — as we will do during the blessing — means saying a visible and public “yes” to the One who died and rose for us; saying “yes” to God who in the humility and weakness of His love is the Almightly, stronger than all the power and intelligence of the world.”
The cross is our seal. We make it with boldness by our fingers on our brow but it doesn’t remain there. It becomes a prayer integrated in everything we do; a prayer over the bread we eat and the cups we drink, in our comings and in goings; before our sleep, when we lie down and when we awake; when we are travelling, and when we are at rest. That was stated by St. Cyril of Jerusalem in his catechetical lecture.
The Sign of the Cross is really a sacred “yes”.