13 “For the Lord is pleasant”. Think not that you faint in praising Him. Your praise of Him is like food: the more ye praise Him, the more ye acquire strength, and He whom you praise becomes the more sweet. “His mercy is everlasting.” For He will not cease to be merciful, after He has freed you: it belongs to His mercy to protect you even unto eternal life. “His mercy,” therefore, “is to everlasting: and His truth from generation to generation”. Understand by “from generation to generation,” either every generation, or in two generations, the one earthly, the other heavenly. Here there is one generation which produces mortals; another which makes such as are everlasting. His Truth is both here, and there. Imagine not that His truth is not here, if His truth were not here, he would not say in another Psalm: “Truth is risen out of the earth;” nor would Truth Itself say, Lo, I am with you always, even unto the end of the world.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)