My covenant will I not break. As the true knowledge of God's mercy can only be obtained from his word, he enjoins us to keep our eyes intently fixed upon his covenant. The more excellent and invaluable a blessing it is, "Never to be rejected after having been once adopted by him," the more difficult it is for us to believe its truth. And we know how many thoughts from time to time present themselves to our minds, tempting us to call it in question. That the faithful, therefore, may not harass themselves beyond measure in debating in their own minds whether or no they are in favor with God, they are enjoined to look to the covenant, and to embrace the salvation which is offered to them in it. God here commends to us his own faithfulness, that we may account his promise sufficient, and that we may not seek the certainty of our salvation any where else. He had said above, If the children of David break my statutes; and now, alluding to that breach, he declares that he will not requite them as they requite him, My covenant will I not break, implying, that although his people may not altogether act in a manner corresponding to their vocation, as they ought to do, he will not suffer his covenant to be broken and disannulled on account of their fault, because he will promptly and effectually prevent this in the way of blotting out their sins by a gratuitous pardon. He is still pursuing the illustration of the preceding proposition, I will not suffer my faithfulness to fail; promising not only to be faithful on his side, as we say, but also that what he has promised shall take full effect, in despite of all the impediments which men may cast in the way; for he will strive against their sins, that by means of them the fruit of his goodness may not be prevented from reaching them. When the Jews, by their ingratitude and treachery, revolted from him, the covenant was not disannulled, because it was founded upon the perfect immutability of his nature. And still, at the present day, when our sins mount even to the heavens, the goodness of God fails not to rise above them, since it is far above the heavens.

My covenant will I not break - literally, I will not pollute, defile, profane. See the notes at Psalm 89:31, where the same word is used. God says that he will not do in regard to the covenant as they had done.
Nor alter the thing - The promise which I have made. I will not make it a different thing. I will not modify its conditions, or withdraw it. It shall stand precisely as it was when I uttered it. What God promises will be exactly performed.

My covenant will I not break - My determination to establish a spiritual kingdom, the head of which shall be Jesus, the son of David, shall never fail. My prophets have declared this, and I will not alter the thing that is gone out of my mouth.

My covenant will I not break, nor (b) alter the thing that is gone out of my lips.
(b) For God in promising has respect to his mercy, and not to man's power in performing.

My covenant will I not break,.... Not the covenant at Sinai, as Aben Ezra, but the covenant of grace made with Christ, and which stands fast with him, Psalm 89:3, which is firm, sure, and stable, and as immovable as mountains and hills, and more so, 2-Samuel 23:5 or "profane" (m) it, though his people profane his statutes, Psalm 89:31, he will not profane his covenant; though they violate his laws, he is a God keeping covenant with them, and will not break his word with them:
nor alter the thing that is gone out of my lips; any promise of his, respecting either the temporal, spiritual, or eternal welfare of his people: or "not change" (n); as he changes not in his nature and perfections, nor in his love and affections, nor in his counsels and purposes; so neither in his covenant and promises, they are always the same, and have a certain and unchangeable accomplishment; there is a performance of whatsoever is spoken by the Lord, Luke 1:45.
(m) "non prophanabo", V. L. Pagninus, Montanus, Tigurine version, &c. (n) "non mutabo", Pagninus, Montanus, Tigurine version, Junius & Tremellius, &c.