The making of the sign of the cross is a very ancient practice. It probably goes back to Apostolic times, and was in common use in the second century. Among the early Christians it was usually made very small, by a slight movement of the finger or thumb, on the forehead or breast. In the days of persecution the faith of the Christian had to be concealed, and any more conspicuous sign would have put him in danger of death.
The devotion to the sign of the cross in those distant days is attested by many writers. They tell us that it was used by the more devout on every occasion. No work was begun without invoking God's blessing by this holy sign. The triple sign of the cross was employed very commonly in the early centuries of the Church and in the Middle Ages. It is not used at present except at the beginning of the Gospels at Mass. It is made by marking the forehead, the lips and the breast with a small cross, using the thumb, and is intended to remind us that our intellect must be attentive to the Word of God, our lips ready to announce His truths, and our hearts filled with love toward Him.
The ordinary method of making the sign of the cross is that which every Catholic learns in early childhood - the putting of the right hand to the forehead, to the breast and to the left and the right shoulder, with the words: "In the name of the Father, and of the Son and of the Holy Spirit. Amen." In past centuries the formula varied greatly. "The sign of Christ." - "The seal of the living God." - " In the name of Jesus." - " In the name of the Holy Trinity." - " Our help is in the name of the Lord," etc., were used. One of these old forms," Oh God, come to my assistance," is still in use at the beginning of [each "hour" or daily section] of the Divine Office..