1 “Give thanks unto the Lord, for He is good, for His mercy endures for ever”. This Psalm contains the praise of God, and all its verses finish in the same way. Wherefore although many things are related here in praise of God, yet His mercy is most commended; for without this plain commendation, he, whom the Holy Spirit used to utter this Psalm, would have no verse be ended. Although after the judgment, by which at the end of the world the quick and the dead must be judged, the just being sent into life eternal, the unjust into everlasting fire, there will not afterwards be those, whom God will have mercy on, yet rightly may His future mercy be understood to be for ever, which He bestows on His saints and faithful ones, not because they will be miserable for ever, and therefore will need His mercy for ever, but because that very blessedness, which He mercifully bestows on the miserable, that they cease to be miserable, and begin to be happy, will have no end, and therefore “His mercy is for ever.”
For that we shall be just from being unjust, whole from being unsound, alive from being dead, immortal from being mortal, happy from being wretched, is of His mercy. But this that we shall be, will be for ever, and therefore “His mercy is for ever.” Wherefore, “give thanks to the Lord;” that is, praise the Lord by giving thanks, “for He is good:” nor is it any temporal good you will gain from this confession, for, “His mercy endures for ever;” that is, the benefit which He bestows mercifully upon you, is for ever.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)