No one knows how old this sign really is. It may well go back to the apostles. Jean Danielou discusses its origins in The Bible and the Liturgy. Suffice to say that for Catholics, prayer begins and ends with this sign of blessing. This sign is where one traces the cross from head to gut, from left shoulder to right shoulder while declaring that the blessing is given in the name of the revealed Trinity. This blessing is entrusted to us before our baptism and traced on our foreheads. As a blessing, it confers identity and mission - whenever we make this sign, we renew our awareness of who we are and the great purpose that God has for us.
The sign itself recalls what Christ has done for us: he died for us and gave his life for our sakes. The death of Christ for our sakes is a dynamic gift, a supreme value, evoking a response of total faith, of conversion to God. Conversion involves a certain kind of death, and a new kind of life: we must die to what naturally motivates us so that we may be moved by God alone.
In addition to the physical sign, we bind ourselves in the name of the Triune God by declaring Him the very source of the blessing. By this declaration, we recall that God has given himself to us, that his very presence lives in our hearts. At the same time, these words recall our baptismal promises. In these promises, we pledge our whole lives to the Father, the Son and the Holy Spirit. This pledge means that we live for the Trinity, that this communion of love has the ultimate call on our existence, that it is the final purpose of our life.
Some are a little squeamish over making such a sign - the sign in fact is an act of surrendering to God the whole of our lives. But for most Christians, especially the persecuted and those who are facing death, this sign is an occasion of hope and a source of strength. For them, there is no other sign which is worthy of Christian prayer. It is the sign of their forefathers - of countless men and women who courageously accepted every trial, persecution, rejection, imprisonment, torture and even death. When we make this sign, we also join ourselves to these holy men and women who went before us -- we enjoy a certain solidarity with them in their sacrfice to God. Their complete trust in God all the way to the end helps us to see that God Father, the Son and the Holy Spirit is worthy of the whole of our lives and the depths of our hearts. Thus, we too find the courage to accept this blessing and to stand firm in our faith.