The Lord hath sworn in truth unto David - He has made a gracious promise, confirmed by an oath, which we may plead in our present necessities. That promise was made “in truth,” that is, sincerely - so that it will certainly be carried out - so that we may appeal to God, on the ground of his faithfulness, to keep his word.
He will not turn from it - We may be certain that he will carry it out. We may appeal to him on the basis of that promise with the utmost confidence.
Of the fruit of thy body will I set upon thy throne - Margin, as in Hebrew, “of thy belly.” The throne would descend to his posterity, 2 Samuel 7:12; see the notes at Psalms 89:3-4.