Hebrews 6:17

"Through this" oath "God desired to show more convincingly to the heirs of the promise" that God's promise, because indeed it is God's, will never be changed. God's oath was infallible in its being interposed, that is, between God, the angel and Abraham.

God, however, ought to have been believed even without an oath: "wherein" (he says) "God willing more abundantly to show unto the heirs of promise the immutability of His counsel, confirmed it by an oath." In these words he comprehends also the believers, and therefore mentions this "promise" which was made to us in common with them. "He mediated" (he says) "by an oath." Here again he says that the Son was mediator between men and God.

When he said above, "the full assurance of the hope," he made clear that God furnished to Abraham "full assurance," not that which belonged to his conduct through works but that which belonged to him through God's redemptive plan.

It was necessary, therefore, he says, to believe in God who had promised without an oath, but since God wanted to do something more, he mediated the promise by an oath.
"to show the firmness of His counsel." He also came to us. For we are the heirs of the promise, being the seed of Abraham; and if we are the seed, then we are also heirs.
"He mediated with an oath." See what he prepares. That the Son was thereafter the mediator of God and men (for he has also become a mediator in the incarnation); and then he mediated with the Father and with Abraham, confirming him with an oath; for the Word was he who both spoke to Abraham and swore. The meaning of the interpretation is well-founded, so that it may be thus. God, that is, the Word, wanting to show the unchangeable nature of those glorifying the Father, mediated with the Father and with Abraham with an oath, that is, he became a mediator in the oath. For through him, as the Word, God and the Father swore.

That is, precisely because an oath gives people full assurance, God also swears. Or, "therefore," instead of: in that He swore, he says, God by Himself, He abundantly proved to us that He will in every way and immutably fulfill what He promised. For although God ought to have been believed even without an oath, nevertheless He condescends for our sake and looks not to His own dignity, but, in order to convince us, permits unworthy things to be said about Himself. For we who are faithful, blessed in his seed, which is Christ, are heirs of the promise. See how even then he says that the Son is the mediator between God and men. For by Himself, as the Word, God and Father swore.

322. – Then when he says, So when God desired to show more abundantly to the heirs of the promise the immutability of his purpose, he interposed an oath, he states the reason why God willed to swear, namely, to show the immutability of His promise. Hence, he says, wherein, i.e., by the fact that He interposed an oath, desiring more abundantly to show; for it was abundant that He promised, but more abundant that He swore, desiring, I say, to show to the heirs of the promise, i.e., of the thing promised: 'They that are the children of the promise are accounted for the seed' (Rom. 9:8), to show, I say, the immutability of his purpose. It should be noted in regard to things which proceed from God that two things must be considered, namely, the coming-forth of things and God's counsel by which such a coming-forth is caused. God's counsel is absolutely unchangeable: 'My counsel shall stand and all my will shall be done' (Is. 46:10). But the disposition is quite changeable, for the Lord sometimes announces something according as the order and coming-forth of things requires, as in Is (38:1) 'Take order with your house, for you shall die and not live'; for the cause of his sickness was such that he should die from it. Also in Jonah (3:4): 'Yet forty days and Nineveh shall be destroyed', because it had deserved destruction: 'I will suddenly speak against a nation and a kingdom, to root out, to pull down and to destroy it. If that nation against which I have spoken shall repent of their evil, I also will repent of the evil I have thought to do to them' (Jer. 18:7). In this case the prophecy is a threat. But sometimes something is announced according to God's eternal counsel, and in that case God never repents or retracts it: 'But the triumpher in Israel will not spare, and will not be moved to repentance' (1 Sam. 15:29). Yet it should be noted that whenever God promises something under oath, it is a prophecy of predestination which reveals God's plan. Such a promise is absolutely unchangeable.