Charles H. Spurgeon on Psalms 89:3

I have made a covenant with my chosen. On heaven's side is God himself, the party proposer. Though he was the party offended, yet the motion for a covenant comes from him...The Father of mercies saith, "The lost creatures cannot contract for themselves; and if another undertake not for them, they must perish; they cannot choose an undertaker for themselves. I will choose one for them, and I will make a covenant with my chosen." On man's side is God's chosen, or chosen One, for the word of God is singular; the Son, the last Adam. Who else as fit to be undertaker on man's side? Who else could have been the Father's choice for this vast undertaking? No angel nor man was capable of it, but the Mighty One (Ps 89:19) whom the Father points out to us as his chosen, Isa 13:1. —Thomas Boston.

Verses 3-4. I made a covenant with my chosen, etc. Do you suppose that this was spoken to David, in his own person only? No, indeed; but to David as the antitype, figure, and forerunner of Jesus Christ. Hence, the Septuagint version renders it, I have covenanted tois eklektois mou with my elect people, or with my chosen ones: i.e. with them in Christ, and with Christ in their name. I have sworn unto David my servant, unto the Messiah, who was typified by David; unto my coeternal Son, who stipulated to take on himself "the form of a servant"; thy seed, i.e. all those whom I have given to thee in the decree of election, all those whom thou shalt live and die to redeem, these will I establish for ever, so as to render their salvation irreversible and inadmissible: and build up thy throne, thy mediatorial throne, as King of saints and covenant Head of the elect, to all generations: there shall always be a succession of favoured sinners to be called and sanctified, in consequence of thy federal obedience unto death; and every period of time shall recompense thy covenant sufferings with an increasing revenue of converted souls, until as many as are ordained to eternal life are gathered in. Observe, here, that when Christ received the promise from the Father concerning the establishment of his (i.e. of Christ's) throne to all generations, the plain meaning is, that his people shall be thus established; for, consider Christ in his divine capacity as the Son of God, and his throne was already established, and had been from everlasting, and would have continued to be established without end, even if he had never been incarnate at all. Therefore, the promise imports that Christ shall reign, not simply as a person in the Godhead (which he ever did, ever will, and ever must); but relatively, mediatorially, and in his office character, as the deliverer and king of Zion. Hence it follows, that his people cannot be lost: for he would be a poor sort of a king who had or might have no subjects to reign over. Consequently, that "throne" of glory on which Christ sits is already encircled in part, and will at last be completely surrounded and made still more glorious, by that innumerable company, that general assembly and church of the firstborn who are written in heaven. —Augustus Montague Toplady.