In the name of the Father,
and of the Son,
and of the Holy Spirit.
As Catholics we begin and end many of our personal prayers, as well as the Mass, with the Sign of the Cross. In the Old Testament we find a prayer called the Shema, a prayer which is said by every devout Jew every day, even until this day. It is in Deuteronomy 6:5 and it says "you shall love the LORD, your God, with all your heart, and with all your soul, and with all your strength" and Jesus tells us in Luke 10:27 "You shall love the Lord, your God, with all your heart, with all your being, with all your strength, and with all your mind."
What we are doing when we make the Sign of the Cross is tracing our on our bodies the cross, the mark of our redemption, and the Shema in the form in which Jesus gave it to us as we touch our mind (forehead), our heart (chest), and our strength (shoulders). We are dedicating these to God in recognition of the promises which were made at our baptism. In addition, we are declaring our acknowledgement that God is a Trinity of persons, the Father the Son and the Holy Spirit. The Sign of the Cross has been used by Christians since early in the 2nd century and possibly since the time of the Apostles themselves.