Once have I sworn by my holiness. God now confirms by an oath what he previously stated he had promised to David; from which it appears that it was not a matter of small importance; it being certain that God would not interpose his holy name in reference to what was of no consequence. It is a token of singular loving-kindness for him, upon seeing us prone to distrust, to provide a remedy for it so compassionately. We have, therefore, so much the less excuse if we do not embrace, with true and unwavering faith, his promise which is so strongly ratified, since in his deep interest about our salvation, he does not withhold his oath, that we may yield entire credence to his word. If we do not reckon his simple promise sufficient, he adds his oath, as it were, for a pledge. The adverb once, denotes that the oath is irrevocable, and that therefore we have not the least reason to be apprehensive of any inconstancy. He affirms that he sware by his holiness, because a greater than himself is not to be found, by whom he could swear. In swearing by Him, we constitute him our judge, and place him as sovereign over us, even as he is our sovereign by nature. It is a more emphatic manner of expression for him to say, by my holiness, than if he had said, by myself, not only because it magnifies and exalts his glory, but also because it is far more fitted for the confirmation of faith, calling back, as it does, the faithful to the earthly habitation which he had chosen for himself, that they might not think it necessary for them to seek him at a distance; for by the term holiness, I have no doubt, he means the sanctuary. And yet he swears by himself, and by nothing else; for, in naming the temple which he had appointed as his seat, he does not depart from himself; but, merely accommodating his language to our rude understandings, swears by his holiness which dwells visibly upon earth. With respect to the elliptical form of the oath, we have seen, in a previous psalm, that this was a manner of swearing quite common among the Hebrews. Thus they were warned that the name of God was not to be used without due consideration, lest, by using it rashly and irreverently, they should draw down upon themselves the Divine vengeance. The abrupt and suspended form of expression was, as it were, a bridle to restrain them, and give them opportunity for reflection. It is no uncommon thing for God to borrow something from the common custom of men.

Once have I sworn by my holiness - That is, once for all; - a single oath - an oath once taken by me - makes it certain. To swear by his "holiness" is to pledge his own holy nature; to make it as certain as that he is holy; to stake the whole question of his holiness on that. That is, If this should not be accomplished - if he should fail in this - it would prove that he was not a holy God.
That I will not lie unto David - Margin, as in Hebrew, "if I lie." The meaning is, He would be found faithful to the promise. See Psalm 89:3-4; compare 2-Samuel 7:8-16.

Once have I sworn - I have made one determination on this head, and have bound myself by my holiness; it is impossible that I should change, and there needs no second oath, the one already made is of endless obligation.

Once have I sworn by my holiness,.... Swearing is ascribed to God after the manner of men, and is done in condescension to the weakness of his people, and to remove doubts and hesitations from them, relating to things spiritual and eternal; as to his everlasting love to them, his covenant with them in Christ, and their perseverance in his grace; and it is made by himself, or one or other of his perfections, as here by his "holiness"; see Amos 4:2, and indeed his holiness being his nature, is no other than he himself, the holy God; and because he could swear by no greater, he sware by himself, that as sure as he was, and was holy, just, and true, he would make good what he promises, Hebrews 6:13 and this is done but once, once for all, that being sufficient; it need not be repeated, nor is it ever revoked; when he swears, he never repents of it, nor changes his mind; and it is to show the immutability of his counsel that he swears at all, Psalm 110:4,
that I will not lie unto David; he will not lie to any, he cannot, it is impossible he should; it would be to deny himself, it is contrary to his being as God, he is not a man that he should lie; it is contrary to his character as the God of truth; he will not lie, neither in his counsel nor covenant, in his purposes nor promises; these are the two immutable things, in which it is impossible he should lie: and he has swore to it that he will not lie to David, to David's son the Messiah, with whom the covenant is made, and stands fast; all the prophecies concerning him he has fulfilled; and all the promises made to him of help and assistance, as man and Mediator, in his work, and of the reward of it, a glory with him, he has made good.

Once--one thing (Psalm 27:4).
by my holiness--as a holy God.
that I will not lie--literally, "if I lie"--part of the form of swearing (1-Samuel 24:6; 2-Samuel 3:35).