2 But I find these two Psalms, the CVth and CVIth so connected, that in one of them, the first, the people of God is praised in the person of the elect, of whom there is no complaint, whom I imagine to have been there in those with whom God was well pleased; but in the following Psalm those are mentioned among the same people who have provoked God; though the mercy of God was not wanting even to these....This Psalm therefore begins like the former; “Confess ye unto the Lord.” But in that Psalm these words follow: “And call upon His Name:” whereas here, it is as follows, “For He is gracious, and His mercy endures for ever”. Wherefore in this passage a confession of sins may be understood; for after a few verses we read, “We have sinned with our fathers, we have done amiss, and dealt wickedly;” but in the words, “For He is gracious, and His mercy endures for ever,” there is chiefly the praise of God, and in His praise confession. Although when any one confesses his sins, he ought to do so with praise of God; nor is a confession of sins a pious one, unless it be without despair, and with calling upon the mercy of God. It therefore does contain His praise, whether in words, when it calls Him gracious and merciful, or in the feeling only, when he believes this....If that mercy be here understood, in respect of which no man can be happy without God; we may render it better, “for ever:” but if it be that mercy which is shown to the wretched, that they may either be consoled in misery, or even freed from it; it is better construed, “to the end of the world,” in which there will never be wanting wretched persons to whom that mercy may be shown. Unless indeed any man ventured to say, that some mercy of God will not be wanting even to those who shall be condemned with the devil and his angels; not a mercy by which they may be freed from that condemnation, but that it may be in some degree softened for them: and that thus the mercy of God may be styled eternal, as exercised over their eternal misery....
3. “Who can express the mighty acts of the Lord?”. Full of the consideration of the Divine works, while he entreats His mercy, “Who,” he says, “can express the mighty acts of the Lord, or make all His praises heard?” We must supply what was said above, to make the sense complete here, thus, “Who shall make all His praises heard?” that is, who is sufficient to make all His praises heard? “Shall make” them “heard,” he says; that is, cause that they be heard; showing, that the mighty acts of the Lord and His praises are so to be spoken of, that they may be preached to those who hear them. But who can make “all,” heard? Is it that as the next words are, “Blessed are they that always keep judgment, and do righteousness in every time”; he perhaps meant those praises of His, which are understood as His works in His commandments? “For it is God,” says the Apostle, “who works in you,”...since He works in these things in a manner that cannot be spoken. “Who will do all His praises heard?” that is, who, when he has heard them, does all His praises? Which are the works of His commandments. As far as they are done, although all which are heard are not performed, He is to be praised, who “works in us both to will and to do of His good pleasure.” For this reason, while he might have said, all His commandments, or, all the works of His commandments; he preferred saying, “His praises.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)