His seed shall endure for ever. There now follows the promise that the right of sovereignty shall always remain with the posterity of David. These two things -- his offspring and his throne, are conjoined; and by these words the everlasting duration of the kingdom is promised, so that it should never pass to those who were of a strange and different race. The sun and the moon are produced as witnesses; for although they are creatures subject to corruption, they yet possess more stability than the earth or air; the elements, as we see, being subject to continual changes. As the whole of this lower world is subject to unceasing agitation and change, there is presented to us a more steadfast state of things in the sun and moon, that the kingdom of David might not be estimated according to the common order of nature. Since, however, this royal throne was shaken in the time of Rehoboam, as we have before had occasion to remark, and afterwards broken down and overthrown, it follows that this prophecy cannot be limited to David. For although at length the outward majesty of this kingdom was put an end to without hope of being re-established, the sun ceased not to shine by day, nor the moon by night. Accordingly, until we come to Christ, God might seem to be unfaithful to his promises. But in the branch which sprung from the root of Jesse, these words were fulfilled in their fullest sense. [1]

Footnotes

1 - "Once. Emphatic. It needs not to be repeated: nor will be." -- Walford.

His seed shall endure forever - His posterity. See the notes at Psalm 89:29. There, the expression is, "his throne as the days of heaven." Here it is, "his throne as the sun before me." The meaning is the same. It would stand through all time. Compare the notes at Psalm 72:5.

His throne as the son - Splendid and glorious! dispensing light, heat, life, and salvation to all mankind.

His seed shall endure for ever,.... This is a confirmation by his oath of what he had before said, Psalm 89:29 which may be understood either of the perseverance of particular believers, of everyone of the spiritual seed of Christ; or of the duration of the church in general, throughout all ages, as before observed; and these being matters of moment and importance, and of which there are sometimes doubts in the minds of the Lord's people about them, and that they may be firmly believed by them, he confirms them with an oath; for God never swears to trivial things; and when he does swear, it is to remove the doubts of his people, and make their minds easy:
and his throne as the sun before me; that is, shall continue as long as it does; see Psalm 89:29, or shall be bright, splendid, and glorious as the sun, so the Targum,
"and his throne light as the sun before me;''
meaning his church and kingdom, of which the throne is an emblem, and which became so in Gospel times, clear and lucid as the sun, Song 6:10, when day was made by the rising of the sun of righteousness, and by the bright shining of the Gospel ministry; and at particular periods since, as in the times of Constantine, when the church was clothed with the sun, and at the Reformation, when Christ appeared with a rainbow on his head, and his face was as the sun, Revelation 12:1 and especially this will be the case of the church in the latter day, when the light of the moon will be as the light of the sun, and the light of the sun seven fold, as the light of seven days; and when the city, the church, will stand in no need of the sun, nor of the moon; and also in the ultimate glory, when the saints will shine as the sun in the kingdom of God; see Isaiah 30:26. This passage is applied to the Messiah by the Jews (a).
(a) Zohar in Genesis. fol. 30. 1.