17 Thus God, determining to show more abundantly to the heirs of promise the immutability of His counsel, confirmed it by an oath,
the immutability of His counsel [reason],
“Immutability” means that God’s counsel does not change. God will not change His promise. He will stand by it. He is true to His own “counsel.” The word “counsel” is the noun form of the word “determining” (verb) earlier in this verse.
The word “confirmed” implies the idea of the action of a mediator between covenanting parties. God placed Himself between who He was and the inheritors of His promise. He lowered Himself to the level of human beings to make Himself a witness to His own oath.
by an oath [pledge, surety],
God’s oath shows that what He promised is unchangeable. He “confirmed” the promise by an oath, which made Himself the guarantor of His promise (He 7:21). The guarantor is the basis for our “anchor” (He 6:19).
God guarantees His own promises.
God made Himself a witness to His own oath. He made this commitment to an oath to show He stood behind His promise. He put His credibility on the line. His condescension to the human level of an oath was to give a guarantee to those who believe on Him. There is no basis for doubt because He has put His character on the line.
God’s promises are infallibly sure because they rest on His oath. His promises are inviolable, so He did not need to make an oath in addition to the promise. He did so to ratify His word. God wanted to make it patently clear about His fidelity to keep His promises.
God’s faithfulness is unchanging. God’s oath is immutable or unchangeable. He never changes His mind. He always acts on the surety He offers. He gave His unconditional covenant to Abraham based on an oath He made with Himself. He gave His oath as a guarantee that His purpose will be fulfilled or accomplished.
God promised the Messiah Christ by an oath.