13 With what fountain then will you be overflowed, and whence runs such a torrent of His Pleasure? “For with You,” says he, “is the fountain of Life.” What is the fountain of Life, but Christ? He came to you in the flesh, that He might bedew your thirsty lips: He will satisfy you trusting, who bedewed you thirsting. “For with You is the fountain of Life; in Your Light shall we see light”. Here a fountain is one thing, light another: there not so. For that which is the Fountain, the same is also Light: and whatever you will you call It, for It is not what you call It: for you can not find a fit name: for It remains not in one name. If you should say, that It is Light only, it would be said to you, Then without cause am I told to hunger and thirst, for who is there that eats light? It is said to me plainly, directly, “Blessed are the pure in heart: for they shall see God.” If It is Light, my eyes must I prepare. Prepare also lips; for That which is Light is also a Fountain: a Fountain, because It satisfies the thirsty: Light, because It enlightens the blind. Here sometimes, light is in one place, a fountain in another. For sometimes fountains run even in darkness; and sometimes in the desert you suffer the sun, findest no fountain: here then can these two be separated: there you shall not be wearied, for there is a Fountain; there you shall not be darkened, for there is Light.
14. “Show forth Your Mercy unto them that know You; Your Righteousness to them that are of a right heart”. As I have said, Those are of a right heart who follow in this life the Will of God. The will of God is sometimes that you should be whole, sometimes that you should be sick. If when you are whole God's Will be sweet, and when you are sick God's Will be bitter; you are not of a right heart. Wherefore? Because you will not make right your will according to God's Will, but wilt bend God's Will to yours. That is right, but you are crooked: your will must be made right to That, not That made crooked to you; and you will have a right heart. It is well with you in this world; be God blessed, who comforts you: it goes hardly with you in this world; be God blessed, because He chastens and proves you; and so will you be of a right heart, saying, “I will bless the Lord at all times: His Praise shall be ever in my mouth.”
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)