I have made a covenant with my chosen. [1] The more effectually to confirm himself and all the godly in the faith of the Divine promise, he introduces God himself as speaking and sanctioning, by his authority, what had been said in the preceding verse. As faith ought to depend on the Divine promise, this manner of speaking, by which God is represented as coming forward and alluring us to himself by his own voice, is more forcible than if the prophet himself had simply stated the fact. And when God in this way anticipates us, we cannot be charged with rashness in coming familiarly to him; even as, on the contrary, without His word we have no ground to presume that he will be gracious to us, or to hope, at the mere suggestion of our own fancy, for what he has not promised. Moreover, the truth of the promise is rendered still more irrefragable, when God declares that he had made a covenant with his servant David, ratified by his own solemn oath. It having been customary in ancient times to engrave leagues and covenants on tables of brass, a metaphor is here used borrowed from this practice. God applies to David two titles of distinction, calling him both his chosen and his servant. Those who would refer the former appellation to Abraham do not sufficiently attend to the style of the Book of Psalms, in which it is quite common for one thing to be repeated twice. David is called the chosen of God, because God of his own good pleasure, and from no other cause, preferred him not only to the posterity of Saul, and many distinguished personages, but even to his own brethren. If, therefore, the cause or origin of this covenant is sought for, we must necessarily fall back upon the Divine election. The name of servant, which follows immediately after, is not to be understood as implying that David by his services merited any thing at the hand of God. He is called God's servant in respect of the royal dignity, into which he had not rashly thrust himself, having been invested with the government by God, and having undertaken it in obedience to his lawful call. When, however, we consider what the covenant summarily contains, we conclude that the prophet has not improperly applied it to his own use, and to the use of the whole people; for God did not enter into it with David individually, but had an eye to the whole body of the Church, which would exist from age to age. The sentence, I will establish thy throne for ever, is partly to be understood of Solomon, and the rest of David's successors; but the prophet well knew that perpetuity or everlasting duration, in the strict and proper sense, could be verified only in Christ. In ordaining one man to be king, God assuredly did not have a respect to one house alone, while he forgot and neglected the people with whom he had before made his covenant in the person of Abraham; but he conferred the sovereign power upon David and his children, that they might rule for the common good of all the rest, until the throne might be truly established by the advent of Christ.

Footnotes

1 - "The word 'mrty, I have said,' is used, in the Book of Psalms, to express two things; either a fixed purpose, or a settled opinion of the person speaking. The Psalmist, therefore, delivers the whole of this second verse in his own person, and introduces not God speaking till the next verse." -- Horsley

I have made a covenant with my chosen - With my chosen one; that is, with David. The original is in the singular number, though by the Septuagint, and the Vulgate, and by Luther, it is rendered in the plural - chosen ones - elect. This is undoubtedly the language of God himself, though it is not expressly ascribed to him. The design is to describe the solemn promise which God had made to David and to his posterity. Compare Psalm 78:70-71. See also, on the use of the phrase "made a covenant," see Psalm 50:5, note; Psalm 83:5, note.
I have sworn unto David my servant - I have taken a solemn oath in regard to him. The substance of the oath is stated in the next verse. The promise referred to is found in 2-Samuel 7:11-16.

I have made a covenant with my chosen - I have made a covenant with Abraham, Isaac, and Jacob; and renewed it with Moses and Joshua in reference to the Israelites in general: but I have made one with David in especial relation to himself and posterity, of whom, according to the flesh, the Christ is to come. And this is the covenant with David: -

(d) I have made a covenant with my chosen, I have sworn unto David my servant,
(d) The prophet shows what the promise of God was, on which he grounded his faith.

I have made a covenant with my chosen,.... Not with Abraham, as the Targum expresses it: but with David, as in the following clause; not David, literally understood, though he was chosen of the Lord to be his servant, and a covenant was made with him, and a promise made to him of the perpetuity of his throne and kingdom in his family, Psalm 78:70 but mystical David, the Messiah, David's son and antitype; after, on this account, called David in Scripture, Ezekiel 34:23 and who is the Lord's "chosen" One, foreordained to be the Redeemer of lost sinners, chosen to be the Mediator between God and them, to be the head of the church, and Saviour of the body; and his human nature was chosen to the grace of union to the Son of God, Psalm 89:19, hence he is called God's elect, Isaiah 43:1 and with him the covenant of grace was made from all eternity, and all the blessings and promises of it were put into his hands; he is the Mediator, surety, and messenger of it, and by his blood it is ratified and confirmed: the Septuagint render it, in the plural number, "with mine elect ones"; and it is a truth, that the covenant of grace is made with all the elect, considered in Christ, and is made with them as such, and not as believers, converted persons, &c. election is the foundation of the covenant, and the source of all covenant blessings:
I have sworn unto David my servant: to the Messiah, called David, as before observed, and who is the Lord's servant, as man and Mediator, of his choosing, calling, sending, and supporting, Isaiah 42:1, to whom he swore, and he will not repent; and which oath of his, joined to his covenant and promise, makes for the strong consolation of the heirs of promise; see Psalm 89:35, the sum and substance of which covenant and oath follow.

The object of this faith expressed in God's words (2-Samuel 7:11-16).
with--or literally, "to"
my chosen--as the covenant is in the form of a promise.

Chosen - With David; whom I have chosen to the kingdom.