2 Let my limbs, he says, serve the Lord: I speak, but it is of Yours I speak. “With my mouth will I make known Your truth:” if I obey not You, I am not Your servant: if I speak on my own part, I am a liar. To speak then from You, and in my own person, are two things: one mine, one Yours: Truth Yours, language mine. Let us hear then what faithfulness he makes known, what mercies he sings.
3. “For You have said, Mercy shall be built up for ever”. It is this that I sing: this is Your truth, for the making known of which my mouth serves. In such wise You say, I build, as not to destroy: for some Thou destroyest and buildest not; and some whom Thou destroyest Thou dost rebuild. For unless there were some who were destroyed to be rebuilt, Jeremiah would not have written, “See, I have this day set you to throw down and to build.” And indeed all who formerly worshipped images and stones could not be built up in Christ, without being destroyed as to their old error. While, unless some were destroyed not to be built up, it would not be written, “He shall destroy them, and not build them up.”... In what follows, he joins these two words, mercy and faithfulness; “For You have said, Mercy shall be built up for ever: Your truth shall be established in the Heavens:” in which mercy and truth are repeated, “for all the ways of the Lord are mercy and truth,” for truth in the fulfilment of promises could not be shown, unless mercy in the remission of sins preceded. Next, as many things were promised in prophecy even to the people of Israel that came according to the flesh from the seed of Abraham, and that people was increased that the promises of God might be fulfilled in it; while yet God did not close the fountain of His goodness even to the Gentiles, whom He had placed under the rule of the Angels, while He reserved the people of Israel as His own portion: the Apostle expressly mentions the Lord's mercy and truth as referring to these two parties. For he calls Christ “a minister of the Circumcision for the truth of God, to confirm the promises made unto the fathers.” See how God deceived not; see how He cast not off His people, whom He foreknew. For while the Apostle is treating of the fall of the Jews, to prevent any from believing them so far disowned of God, that no wheat from that floor's fanning could reach the granary, he says, “God has not cast away His people, whom He foreknew; for I also am an Israelite.” If all that nation are thorns, how am I who speak unto you wheat? So that the truth of God was fulfilled in those Israelites who believed, and one wall from the circumcision is thus brought to meet the corner stone. But this stone would not form a corner, unless it received another wall from the Gentiles: so that the former wall relates in a special manner to the truth, the latter to the mercy of God. “Now I say,” says the Apostle, “that Jesus Christ was a minister of the Circumcision for the truth of God, to confirm the promise made unto the fathers: and that the Gentiles might glorify God for His mercy.” Justly then is it added, “Your truth shall Thou establish in the Heavens:” for all those Israelites who were called to be Apostles became as Heavens which declare the glory of God: as it is written by them, “The Heavens declare the glory of God, and the firmament shows His handywork.”...Since, although they were taken up from hence before the Church filled the whole world, yet as “their words reached to the ends of the world,” we are right in supposing this which we have just read, “Your truth shall Thou establish in the Heavens,” fulfilled in them.
4. “You have said, I have made a covenant with My chosen”. What covenant, but the new, by which we are renewed to a fresh inheritance, in our longing desire and love of which we sing a new song. “I have made a covenant with My chosen,” says the Psalmist: “I have sworn unto David My servant.” How confidently does he speak, who understands, whose mouth serves truth! I speak without fear; since “You have said.” If You make me fearless, because You have said, how much more so dost Thou make me, when You have sworn! For the oath of God is the assurance of a promise. Man is justly forbidden to swear: lest by the habit of swearing, since a man may be deceived, he fall into perjury. God alone swears securely, because He alone is infallible.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)