Wherein God - On account of which; or since an oath had this effect, God was willing to appeal to it in order to assure his people of salvation.
Willing more abundantly - In the most abundant manner, or to make the case as sure as possible. It does not mean more abundantly than in the case of Abraham, but that he was willing to give the most ample assurance possible. Coverdale renders it correctly, “very abundantly.”
The heirs of promise - The heirs to whom the promise of life pertained; that is, all who were interested in the promises made to Abraham - thus embracing the heirs of salvation now.
The immutability of his counsel - His fixed purpose. He meant to show in the most solemn manner that his purpose would not change. The plans of God never change; and all the hope which we can have of heaven is founded on the fact that his purpose is immutable. If he changed his plans; if he was controlled by caprice; if he willed one thing today and another thing tomorrow, who could confide in him, or who would have any hope of heaven? No one would know what to expect; and no one could put confidence in him. The farmer plows and sows because he believes that the laws of nature are settled and fixed; the mariner ventures into unknown seas because the needle points in one direction; we plant an apple tree because we believe it will produce apples, a peach because it will produce peaches, a pear because it will produce a pear. But suppose there were no settled laws, that all was governed by caprice; who would know what to plant? Who then would plant anything? So in religion. If there were nothing fixed and settled, who would know what to do? If God should change his plans by caprice, and save one man by faith today and condemn another for the same faith tomorrow; or if he should pardon a man today and withdraw the pardon tomorrow, what security could we have of salvation? How grateful, therefore, should we be that God has an “immutable counsel,” and that this is confirmed by a solemn oath! No one could honor a God that had not such an immutability of purpose; and all the hope which man can have of heaven is in the fact that He is unchanging.
Confirmed it by an oath - Margin, “Interposed himself.” Tyndale and Coverdale, “added an oath.” The Greek is, “interposed with an oath” - ἐμεσιτεύσεν ὅρκῳ emesiteusen horkō. The word used here - μεσιτεύω mesiteuō - means to mediate or intercede for one; and then to intervene or interpose. The meaning here is, “that he interposed an oath” between himself and the other party by way of a confirmation or pledge.