John Calvin Commentary Psalms 89:28

"My lovingkindness will I keep for him for evermore; And my covenant shall stand fast with him." — Psalms 89:28 (ASV)

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 forever. 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 point is, that God will always be merciful to David, so 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 forever on the royal throne.

Since there is nothing under heaven that is long-lasting, the days of heaven is an expression used to denote everlasting duration. From this it follows that this prophecy cannot have its full accomplishment in anyone until we come to Christ, in whom alone, in the strict and proper sense, this everlasting duration is to be found.