3 But wherefore is there hope? “For there is propitiation with You”. And what is this propitiation, except sacrifice? And what is sacrifice, save that which has been offered for us? The pouring forth of innocent blood blotted out all the sins of the guilty: so great a price paid down redeemed all captives from the hand of the enemy who captured them. “With You,” then, “there is propitiation.” For if there were not mercy with You, if Thou chosest to be Judge only, and refused to be merciful, You would mark all our iniquities, and search after them.
Who could abide this? Who could stand before You, and say, I am innocent? Who could stand in Your judgment? There is therefore one hope: “for the sake of Your law have I borne You, O Lord.” What law? That which made men guilty. For a “law, holy, just, and good,” was given to the Jews; but its effect was to make them guilty. A law was not given that could give life, but which might show his sins to the sinner. For the sinner had forgotten himself, and saw not himself; the law was given him, that he might see himself.
The law made him guilty, the Lawgiver freed him: for the Lawgiver is the Supreme Power....There is therefore a law of the mercy of God, a law of the propitiation of God. The one was a law of fear, the other is a law of love. The law of love gives forgiveness to sins, blots out the past, warns concerning the future; forsakes not its companion by the way, becomes a companion to him whom it leads on the way. But it is needful to agree with the adversary, while you are with him in the way. For the Word of God is your adversary, as long as thou dost not agree with it.
But you agree, when it has begun to be your delight to do what God's Word commands. Then he who was your adversary becomes your friend: so, when the way is finished, there will be none to deliver you to the Judge. Therefore, “For the sake of Your law I have waited for You, O Lord,” because you have condescended to bring in a law of mercy, to forgive me all my sins, to give me for the future warnings that I may not offend....“For the sake,” therefore, “of” this “law I have waited for You, O Lord.” I have waited until You may come and free me from all need, for in my very need You have not forsaken the law of mercy....“My soul has waited for Your word.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)