4 “In this way, wherein I was walking, they hid a trap for me.” This “way wherein I was walking,” is Christ; there have they laid a trap for me, who persecute me in Christ, for Christ's Name's sake. There then “have they hid for me a trap.” What in me do they hate, what in me do they persecute? That I am a Christian....For the heretics too wish to hide a stumbling-block for us in the Name of Christ, and are themselves deceived. What they think that they put in the way, they put outside the way, for they themselves are outside the way.
They cannot set a trap where themselves are not....The Pagan thinks to put a stumbling-block in the way, when he says to me, “You worship a crucified God.” He finds fault with the Cross of Christ, which he understands not. He thinks that he sets in Christ, what he sets near the way. I will not depart from Christ, so shall I not fall from the way into the trap. Let him mock at Christ crucified, let me see the Cross of Christ on the foreheads of kings. What he laughs at, therein am I saved.
Nought is prouder than a sick man, who laughs at his own medicine. If he laughed not at it, he would take it, and be healed. The Cross is the sign of humility, but he through excess of pride acknowledges not that whereby may be healed the swelling of his soul. But if I acknowledge, I am walking in the way. So far am I from blushing at the Cross, that in no secret place do I keep the Cross of Christ, but bear it on my forehead. Many sacraments we receive, one in one way another in another: some as you know we receive with the mouth, some we receive over the whole body.
But because the forehead is the seat of the blush of shame, He who said, “Whosoever shall be ashamed of Me before men, of him will I be ashamed before My Father which is in heaven,” set, so to speak, that very ignominy which the Pagans mock at, in the seat of our shame. You hear a man assail a shameless man and say, “He has no forehead.” What is, “He has no forehead”? He has no shame. Let me not have a bare forehead, let the Cross of my Lord cover it....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)