Psalms 89:28

28.And I will keep my mercy to him for ever. We see how God frequently repeats, that he had set up the kingdom of David with the express design of establishing it for ever. By placing his mercy first in order, and then adding his covenant, he points out the cause of this covenant, intimating in one word, that it is gratuitous, and that his grace is not only the foundation on which it rests, but also the cause why it is preserved inviolate. The amount is, that God will be always merciful to David, in order that his covenant may never fail. From this it follows, that its inviolability depends upon the mere good pleasure of God. In the next verse, God expresses the effect of his truth, declaring, that the posterity of David will sit for ever on the royal throne. There being nothing under heaven of long continuance, the days of heaven is an expression employed to denote everlasting duration. Whence it follows, that this prophecy cannot have its full accomplishment in any till we come to Christ, in whom alone, in the strict and proper sense, this everlasting duration is to be found.