Of promise.--Rather, of the promise. The promise made to Abraham was substantially and really (see Hebrews 6:13) that which embraced all Messianic hope; of this promise not Abraham's sons only, but all "they which are of faith" (Galatians 3:7; Galatians 3:29), Abraham's spiritual seed, are the heirs. In an Epistle so distinctly Pauline there can be no doubt as to this interpretation.
Confirmed it by an oath.--Literally, mediated with an oath. When a man confirms a promise or declaration to another by solemn appeal to God, between the two God is Mediator. Condescending to man's weakness, that the certainty may be "more abundant," God. thus confirms His word, at once the Promiser and the Mediator: God the Promiser (if we may so speak) makes appeal to God the Hearer and Witness of the oath. We cannot doubt, as we read this whole passage, that there is a special reason for the emphasis thus laid on God's oath to Abraham. The writer dwells on this confirmation of the divine word of promise, not merely because it is the first recorded in sacred history, but because he has in thought the declaration of Psalm 110:4. To this as yet he makes no reference; though he has quoted from the verse repeatedly, it has been without mention of the divine oath: but throughout the section before us he is preparing the way for his later argument in Hebrews 7:21.
more abundantly to show unto the heirs of promise the immutability of his counsel; by which is meant, not the Gospel nor the ordinances of it, though these are sometimes called the counsel of God; but the decree of God, concerning the salvation of his people by Jesus Christ, which is immutable; as appears from the unchangeableness of his nature, the sovereignty of his will, the unsearchableness of his wisdom, the omnipotence of his arm, and the unconditionality of the thing decreed, and from that and the purpose of it being in Christ: and the immutability of this, God was willing to show "more abundantly" than in other purposes, though all God's purposes are unchangeable; or than had been shown to the Old Testament saints; and more than was necessary, had it not been for man's weakness: even to "the heirs of promise"; not any earthly temporal promise, but the promise of grace and glory; the promise of eternal life; the heirs of which are not only Abraham, Isaac, and Jacob, or God's elect among the Jews, but all that are Christ's; who are justified by his righteousness, believe in him, and are the children of God; for as many as are such are heirs of eternal glory, and of the promise of it: and that the unchangeableness of God's purpose in saving them by Christ might be manifest to them, and be out of all doubt, he "confirmed it by an oath"; his counsel and purpose; he not only determined in his mind that he would save them, and promised it in his covenant; but he also, to confirm it the more to the persons concerned in it, if possible, annexed his oath to it; or "he interposed or acted the part of a Mediator by an oath"; which some refer to Christ's mediation between God and Abraham, when he swore unto him, as before observed; but rather it expresses the interposition of the oath between God's purpose and promise, and man's weakness: God did as it were bind himself by his oath, or lay himself under obligation, or become a surety, for the fulfilment of his purpose and promise; which shows the super-abounding grace of God, the weakness of man, and what reason the heirs of promise have to believe.