4 “Praise the Lord in His saints,” that is, in those whom He has glorified: “praise Him in the firmament of His power”. “Praise Him in His deeds of strength;” or, as others have explained it, “in His deeds of power: praise Him according to the multitude of His greatness”. All these His saints are; as the Apostle says, “But we may be the righteousness of God in Him.” If then they be the righteousness of God, which He has wrought in them, why are they not also the strength of Christ which He has wrought in them, that they should rise again from the dead?
For in Christ's resurrection, “strength” is especially set forth to us, for in His Passion was weakness, as the Apostle says. And well does it say, “the firmament of His power.” For it is the “firmament of His power” that He “dies no more, death has no more dominion over Him.” Why should not they also be called “the works of” God's “strength,” which He has done in them: yea rather, they themselves are the works of His strength; just as it is said, “We are the righteousness of God in Him.”
For what more powerful than that He should reign for ever, with all His enemies put under His feet? Why should not they also be “the multitude of His greatness”? Not that whereby He is great, but whereby He has made them great, many as they are, that is, thousands of thousands. Just as righteousness too is understood in two ways, that whereby He is righteous, and that which He works in us, so as to make us His righteousness. These same saints are signified by all the musical instruments in succession, to praise God in. For what the Psalmist began with, saying, “Praise the Lord in His saints,” that he carries out, signifying in various ways these same saints of His.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)