15 “Let not the foot of pride come against me”. But now he said, The children of men shall put their trust under the shadow of Your wings: they shall be satiated with the fullness of Your House. When one has begun to be plentifully overflowed with that Fountain, let him take heed lest he grow proud. For the same was not wanting to Adam, the first man: but the foot of pride came against him, and the hand of the sinner removed him, that is, the proud hand of the devil.
As he who seduced him, said of himself, “I will sit in the sides of the north;” so he persuaded him, by saying, “Taste, and you shall be as gods.” By pride then have we so fallen as to arrive at this mortality. And because pride had wounded us, humility makes us whole. God came humbly, that from such great wound of pride He might heal man. He came, for “The Word was made Flesh, and dwelt among us.” He was taken by the Jews; He was reviled of them. You heard when the Gospel was read, what they said, and to Whom they said, “You have a devil:” and He said not, You have a devil, for you are still in your sins, and the devil possesses your hearts.
He said not this, which if He had said, He had said truly: but it was not meet that He should say it, lest He should seem not to preach Truth, but to retort evil speaking. He let go what He heard as though He heard it not. For a Physician was He, and to cure the madman had He come. As a Physician cares not what he may hear from the madman; but how the madman may recover and become sane; nor even if he receive a blow from the madman, cares he; but while he to him gives new wounds, he cures his old fever: so also the Lord came to the sick man, to the madman came He, that whatever He might hear, whatever He might suffer, He should despise; by this very thing teaching us humility, that being taught by humility, we might be healed from pride: from which he here prays to be delivered, saying, “Let not the foot of pride come against me; neither let the hand of the sinner remove me.”
For if the foot of pride come, the hand of the sinner removes. What is the hand of the sinner? The working of him that advises ill. Have you become proud? Quickly he corrupts you who advises ill. Humbly fix yourself in God, and care not much what is said to you. Hence is that which is elsewhere spoken, “From my secret sins cleanse Thou me; and from others' sins also keep Your servant.” What is, “From my secret sins”? “Let not the foot of pride come against me.” What is, “From other men's sins also keep Your servant”? “Let not the hand of the wicked remove me.” Keep that which is within, and you shall not fear from without.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)