1....The Psalm which we have just sung is in many parts somewhat obscure. When by the help of the Lord what has been said shall begin to be expounded and explained, you will see that you are hearing things which you knew already. But for this cause are they said in manifold ways, that variety of expression may remove all weariness of the truth....
2. “Lord, I have cried unto You, hear Thou me”. This we all can say. This not I alone say: whole Christ says it. But it is said rather in the name of the Body: for He too, when He was here and bore our flesh, prayed; and when He prayed, drops of blood streamed down from His whole Body. So is it written in the Gospel: “Jesus prayed earnestly, and His sweat was as it were great drops of blood.” What is this flowing of sweat from His whole Body, but the suffering of martyrs from the whole Church? “Listen unto the voice of my prayer, while I cry unto You.” You thought the business of crying already finished, when you said, “I have cried unto You.” You have cried; yet think not yourself safe. If tribulation be finished, crying is finished: but if tribulation remain for the Church, for the Body of Christ, even to the end of the world, let it not only say, “I have cried unto You,” but also, “Listen unto the voice of my prayer.”
3. “Let my prayer be set forth in Your sight as incense, and the lifting up of my hands an evening sacrifice”. That this is wont to be understood of the Head Himself, every Christian acknowledges. For when the day was now sinking towards evening, the Lord upon the Cross “laid down His life to take it again,” did not lose it against His will. Still we too are figured there. For what of Him hung upon the tree, save what He took of us? And how can it be that the Father should leave and abandon His only begotten Son, especially when He is one God with Him? Yet, fixing our weakness upon the Cross, where, as the Apostle says, “our old man is crucified with Him,” He cried out in the voice of that our “old man,” “Why have You forsaken Me?” That then is the “evening sacrifice,” the Passion of the Lord, the Cross of the Lord, the offering of a salutary Victim, the whole burnt offering acceptable to God. That “evening sacrifice” produced, in His Resurrection, a morning offering. Prayer then, purely directed from a faithful heart, rises like incense from a hallowed altar. Nought is more delightful than the odour of the Lord: such odour let all have who believe.
4....“Set, O Lord, a watch before my mouth, and a door of restraint around my lips”. He said not a barrier of restraint, but “a door of restraint.” A door is opened as well as shut. If then it be a “door,” let it be both opened and shut; opened, to confession of sin; closed, to excusing sin. So will it be a “door of restraint,” not of ruin. For what does this “door of restraint” profit us? What does Christ pray in the name of His Body? “That Thou turn not aside My heart to wicked words”. What is, “My heart”? The heart of My Church; the heart, that is, of My Body....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)