Apostolic Tradition 37

1But imitate him always, by signing thy forehead sincerely; for this is the sign of his Passion, manifest and approved against the devil if so thou makest it from faith; not that thou mayest appear to men, but knowingly offering it as a shield. For the adversary, seeing its power coming from the heart, that a man displays the publicly formed image of baptism, is put to flight; not because thou spittest, but because the Spirit in thee breathes him away. When Moses formed it by putting the blood of the Paschal lamb that was slain on the lintel and anointing the side-posts, he signified the faith which now we have in the perfect Lamb.