Your seed will I establish forever, and build up your throne to all generations.
Read Chapter 89
Augustine of Hippo
5. Let us see then what God hath sworn. "I have sworn," He saith, "to David My servant; thy seed will I establish for ever" (ver. 4). But what is the seed of David, but that of Abraham. And what is the seed of Abraham? "And to thy seed," He saith, "which is Christ." But perhaps that Christ, the Head of the Church, the Saviour of the body, is the seed of Abraham, and therefore of David; but we are not Abraham's seed? We are assuredly; as the Apostle saith, "And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise." In this sense, then, let us take the words, brethren, "Thy seed will I stablish for ever," not only of that Flesh of Christ, born of the Virgin Mary, but also of all of us who believe in Christ, for we are limbs of that Head. This body cannot be deprived of its Head: if the Head is in glory for ever, so are the limbs, so that Christ remains entire for ever. "Thy seed will I stablish for ever: and set up thy throne to generation and generation." We...
Generation. David's posterity occupied the throne for a long time, (Haydock) and subsisted till the coming of Christ; so that if any conqueror of that family had then appeared, the Jews would not have hesitated to admit, that this prediction was fulfilled. It is there misfortune to understand the text in this sense, whereas God spoke of the spiritual kingdom of his Son, which is to be perpetual. They can never answer the argument which the Fathers urged in the 4th century, and which has attained fresh strength from the longer duration of misery under which the royal family of David has been depressed. It is plain, that it has enjoyed no power from many ages, and as God's word is invariable, He could not have promised an everlasting earthly dominion. (Berthier)
The temporal kingdom of David decayed at the captivity, and is now wholly destroyed. But Christ was of this family, and established the Church, his spiritual kingdom, which shall continue unto the end. (Worthington)