As early as the ninth century, the faithful were making the sign of the cross on their forehead and breast at the reading of the Gospel. Then, in the 11th century, we find the deacon and the faithful making the sign of the cross on the forehead, mouth, and breast after the words "A reading of the holy gospel . . ." (Jungmann, Mass of the Roman Rite).
According to Jungmann,
The original idea of this signing of oneself is probably indicated in the scriptural text frequently cited in this connection, the quotation about the wicked enemy who is anxious to take the seed of the word of God away from the hearts of hearers (Mark 4:15). . . . But another explanation takes over by degrees; an ever-increasing stress is placed on the readiness to acknowledge God’s word with courage. . . . The meaning is this: For the word which Christ brought and which is set down in this book we are willing to stand up with a mind that is open, we are ready to confess it with our mouth, and above all we are determined to safeguard it faithfully in our hearts. (CIN-Origin of the Sign of the Cross, Father Mateo, July 28, 1991)