5 Unto whom is it said, “O you seed of Abraham His servant, you children of Jacob, His chosen”?....He next adds, “He is the Lord our God: His judgments are in all the world”. Is He the God of the Jews only? God forbid! “He is the Lord our God:” because the Church, where His judgments are preached, is in all the world....
6. “He has been always mindful of His covenant”. Other copies read, “for evermore;” and this arises from the ambiguity of the Greek. But if we are to understand “alway” of this world and not of eternity, why, when he explains what covenant He was mindful of, does he add, “The word that He made to a thousand generations”? Now this may be understood with a certain limitation; but he afterwards says, “Even the covenant that He made with Abraham”: “and the oath that He swore unto Isaac; and appointed the same unto Jacob for a law, and to Israel for an everlasting testament”. But if in this passage the Old Testament is to be understood, on account of the land of Canaan; for thus the language of the Psalm runs, “saying, Unto you will I give the land of Canaan: the lot of your inheritance”: how is it to be understood as everlasting, since that earthly inheritance could not be everlasting? And for this reason it is called the Old Testament, because it is abolished by the New. But a thousand generations do not seem to signify anything eternal, since they involve an end; and yet are also too numerous for this very temporal state. For by howsoever few years a generation is limited, such as in Greek is called γεν·α, whereof the shortest period some have fixed is at fifteen years, after which period man has the power of generation; what then are those “thousand generations,” not only from the time of Abraham, when that promise was made him, unto the New Testament, but from Adam himself down to the end of the world? For who would dare to say that this world should last for 15000 years? Hence it seems to me that we ought not to understand here the Old Testament, which it said through the prophet was to be cancelled by the New: “Behold, the days come, says the Lord, when I will make a new covenant.”...After saying, “He has been mindful of His covenant unto an age;” which we ought to understand as lasting for evermore, the covenant, namely, of justification and an eternal inheritance, which God has promised to faith; he adds, “and the Word that He commanded unto a thousand generations.” What means “commanded”?...The command then was faith, that the righteous should live by faith; and an eternal inheritance is set before this faith. “A thousand generations,” then, are, on account of the perfect number, to be understood for all; that is, as long as generation succeeds generation, so long is it commanded to us to live by faith. This the people of God does observe, the sons of promise who succeed by birth, and depart by death, until every generation be finished; and this is signified by the number thousand; because the solid square of the number ten, ten times ten, and this taken ten times amounts to a thousand. “Even the covenant,” he says, “which He made with Abraham: and the oath that He swore unto Isaac; and appointed the same unto Jacob,” that is, Jacob himself, “for a law.” These are the very three patriarchs, whose God He calls Himself in a special sense, whom the Lord also does name in the New Testament, where He says, “Many shall come from the east and the west, and shall sit down with Abraham, and Isaac, and Jacob, in the kingdom of heaven.” This is everlasting inheritance....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)