John Calvin Commentary Hebrews 6:17

"Wherein God, being minded to show more abundantly unto the heirs of the promise the immutability of his counsel, interposed with an oath;" — Hebrews 6:17 (ASV)

God, willing, etc. See how kindly God, as a gracious Father, accommodates himself to our slowness to believe; as he sees that we do not rest on his simple word, he adds an oath so that he might more fully impress it on our hearts. From this, it also appears how important it is for us to know that there is such certainty regarding his goodwill towards us that there is no longer any occasion for wavering or trembling.

For when God forbids his name to be taken in vain or on a slight occasion, and denounces the severest vengeance on all who rashly abuse it, and when he commands reverence to be given to his majesty, he thus teaches us that he holds his name in the highest esteem and honor.

The certainty of salvation is, then, a necessary thing; for he who forbids swearing without reason has been pleased to swear for the sake of making it certain. And from this we may also conclude how highly he values our salvation; for in order to secure it, he not only pardons our unbelief but, giving up as it were his own right and yielding to us far more than what we could claim, he kindly provides a remedy for it.

Unto the heirs of promise, etc. He seems especially to point out the Jews; for though the heirship eventually came to the Gentiles, yet the former were the first lawful heirs, and the latter, being aliens, were made the second heirs, and that beyond the right of nature. So Peter, addressing the Jews in his first sermon, says,

To you and to your children is the promise made, and to those who are afar off, whom the Lord shall call (Acts 2:39).

He indeed left a place for adventitious heirs, but he sets the Jews in the first rank, according to what he also says in the third chapter, Ye are the children of the fathers and of the covenant, etc. (Acts 3:25). So also in this place, the Apostle, in order to make the Jews more ready to receive the covenant, shows that it was chiefly for their sakes it was confirmed by an oath. At the same time, this declaration belongs to us also today, for we have entered into the place left by them through unbelief.

Observe that what is testified to us in the Gospel is called the counsel of God, so that no one may doubt that this truth proceeds from the very inmost thoughts of God. Believers, therefore, ought to be fully persuaded that whenever they hear the voice of the Gospel, the secret counsel of God, which was hidden in him, is proclaimed to them. Thus, what he has decreed concerning our salvation before the creation of the world is made known to them.