What is the origin of the sign of the Cross?

The sign of the cross is foreshadowed in the prophecy of Ezekiel: “Go through the city, through Jerusalem, and put a mark upon the foreheads of those who sigh and groan over all the abominations committed in it… but touch no one upon whom is the mark” (Ezk 9:4–6; cf. Rev 7:3; see Q298, Q307). The “mark” of Ezekiel is, in Hebrew, the letter taw, which in the ancient Hebrew script was written in the form of a cross: the sealing of the faithful on the forehead with the sign of the cross is thus already inscribed in the prophets.

As an outward sign of sealing and blessing, the sign of the cross has apostolic origins, attested in the rite of baptism as early as the second century. It is in itself a summary of the saving faith, that we are redeemed and reconciled to God “through the blood of his cross” (Col 1:20; see Q012, Q195). In the customary manner of making it, the three fingers joined together represent the Holy Trinity, one in essence and undivided, and the two folded fingers the two natures of Christ, fully divine and fully human.