I am often asked: What does the sign of the cross mean? What does it mean to cross oneself?
I want to say: “The sign of the cross is a prayer.” But, if it is a prayer, what sort of prayer? Is it always a prayer?
Making the sign of the cross is — or, better, can be — a prayer to be made one with Christ crucified. It is — it can be — an invocation of the Holy Trinity, a wordless confession of faith in the loving God.
Why this can be? Does the bare gesture alone not suffice as prayer? Or do I need to mean it? To misquote Stanley Cavell: Must we mean what we pray?
When I cross myself, must I intend to pray for the gesture to count as a prayer? In what circumstance could the gesture itself be a prayer? If I cross myself when I am absentminded or preoccupied or inattentive, do I pray? What about when I am at a loss for words, when exhaustion or fear or joy has overwhelmed me? What would it mean then, to make the sign of the cross beyond words? — “We do not know how to pray as we ought, but the Spirit himself intercedes for us with sighs too deep for words” (Rom. 8:26).
“And the Lord said to him, “Go through the city, through Jerusalem, and put a mark upon the foreheads of the men who sigh and groan over all the abominations that are committed in it” (Ezek. 9:4).
John Donne, in one of his sermons, comments briefly on this passage. Following an ancient reading, he understands this mark to be the Hebrew letter Tav. He thinks the reason the Lord used this sign is that the Tau has the shape of a cross, so that the mark on “the foreheads of the men who sigh and groan” becomes a sign of the cross of Christ. Donne writes:
God imprinted upon them, that sighed, and mourned, that Tau, that letter, which had the form of the Cross, that it might be an evidence, that all their crosses shall be swallowed up in his Cross, their sighs in his sighs, and their agonies in his.
In other words, Donne understands this cross-shaped sign in sacramental terms. For “an evidence” is something obvious to the eye, a material sign — perhaps even “an outward and visible sign of an inward and spiritual grace.” Applying Donne’s suggestion to the sign of the cross, we can understand it, too, as an evidence, a protestation, a solemn declaration, of our hope in Christ Jesus. The sign of the cross is given to us as a witness that all our crosses shall be swallowed up in his Cross, our sighs in his sighs, and our agonies in his.
Thomas Traherne on the Cross:
The Cross is the abyss of wonders, the centre of desires, the school of virtues, the house of wisdom, the throne of love, the theatre of joys, and the place of sorrows; It is the root of happiness, and the gate of Heaven.
Of all the things in Heaven and Earth it is the most peculiar. It is the most exalted of all objects. It is an Ensign lifted up for all nations, to it shall the Gentiles seek, His rest shall be glorious: the dispersed of Judah shall be gathered together to it, from the four corners of the earth. If Love be the weight of the Soul, and its object the centre, all eyes and hearts may convert and turn unto this Object: cleave unto this centre, and by it enter into rest. There we might see all nations assembled with their eyes and hearts upon it. There we may see God’s goodness, wisdom and power: yea His mercy and anger displayed. There we may see man’s sin and infinite value. His hope and fear, his misery and happiness. There we might see the Rock of Ages, and the Joys of Heaven. There we may see a Man loving all the world, and a God dying for mankind. There we may see all types and ceremonies, figures and prophecies. And all kingdoms adoring a malefactor: An innocent malefactor, yet the greatest in the world. There we may see the most distant things in Eternity united: all mysteries at once couched together and explained. The only reason why this Glorious Object is so publicly admired by Churches and Kingdoms, and so little thought of by particular men, is because it is truly the most glorious: It is the Rock of Comforts and the Fountain of Joys. It is the only supreme and sovereign spectacle in all Worlds. It is a Well of Life beneath in which we may see the face of Heaven above: and the only mirror, wherein all things appear in their proper colours: that is, sprinkled in the blood of our Lord and Saviour.
The sign of the cross can mean all this, and more.
 John Donne, “Sermon 7: Preached to the King at White-hall, upon the occasion of the Fast, April 5. 1628,” lines 194-200, in Sermons Preached at the Court of Charles I, Vol. 3 of The Oxford Editions of the Sermons of John Donne, ed. David Colclough (Oxford, 2013). I have modernized the spelling.