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.

My mercy will I keep for him for evermore - I will not withdraw my favor from him, nor from his posterity, Psalm 89:33-36. In him, and in his Great Descendant, the throne shall be established forever. This dominion will not be like the changing dynasties of this world, but will be perpetual and eternal.
And my covenant shall stand fast with him - See 2-Samuel 7:14-16; 2-Samuel 23:5. It shall be firm, or established with him and his family.

My mercy will I keep for him for evermore,.... That is, for his mystical body, his church and people; for whom stores of mercy are kept with him, to be laid out in their regeneration, pardon, salvation, and eternal life; for to them the mercy of God is from everlasting to everlasting, Psalm 103:17, unless this is to be understood of the "grace" and "kindness" (g) of God, as the word may be rendered; his free favour and love to Christ, which always continues; for as he was always his dearly beloved Son, that lay in his bosom from eternity, so he continued, throughout his state, of humiliation, his well beloved, in whom he was well pleased, and still is, and ever will:
and my covenant shall stand fast with him; being made with him as the head and representative of his people, it remains, and will remain, sure, firm, and immoveable; its blessings are "sure mercies", and its promises are all "yea and amen in Christ": the stability of it, and of all that is in it, is owing to its being made with him, and being in his hands, who is the surety, Mediator, and messenger of it.
(g) "bonitatem meam", Musculus, Tigurine version; "benignitatem meam", Junius & Tremellius, Piscator; "gratiam meam", Cocceius, Gejerus, Michaelis.

This relation is perpetual with David's descendants, as a whole typical in official position of his last greatest descendant. Hence though in personal relations any of them might be faithless and so punished, their typical relation shall continue. His oath confirms His promise, and the most enduring objects of earth and heaven illustrate its perpetual force (Psalm 72:5, Psalm 72:7, Psalm 72:17).