If you are tempted, seal (that is, making the sign of the cross) your foreheads reverently. For this is the Sign of the Passion, displayed and made manifest against the devil, provided that you do it with faith, not to be seen by men, but by presenting it with skill like a shield.
Because the Adversary, when he sees the strength of the heart and when he sees the inner man which is animated by the Word show, formed on the exterior, the interior image of the Word, he is made to flee by the Spirit which is in you. This is symbolized by the Paschal lamb which was sacrificed, the blood of which Moses sprinkled on the threshold, and smeared on the doorposts. He told us of the faith which is now in us, which was given to us through the perfect Lamb.
By sealing the forehead and eyes with the hand, we turn aside the one who is seeking
to destroy us.
Hippolytus, Apostolic Tradition, chapter 42.