Hebrews 6:13

For when God made promise to Abraham
The apostle proposes Abraham as a pattern, because he was the father of these Hebrews, and of all believers; and because they were interested in the promise made to him, and had a right to the same blessing with him; and because he was remarkable for his faith and patience: the promise made to him is not that in ( Genesis 12:1-3 ) nor that in ( Genesis 15:1 Genesis 15:4 Genesis 15:5 Genesis 15:13 Genesis 15:14 ) but that in ( Genesis 22:16 Genesis 22:17 ) for that only had an oath annexed to it: and this was made by Jesus Christ, there called the Angel of the Lord, and here God; and who is truly and properly so; and than whom there is none greater; and who elsewhere, as here, is said to swear by himself, ( Isaiah 45:23 ) as follows:

because he could swear by no greater, he sware by himself;
swearing is ascribed to a divine person after the manner of men, and in condescension to them; and who is never introduced swearing, but in matters of moment and of great importance; the note of Philo the Jew

``well does he (God) confirm the promise with an oath, and with an oath that becomes God; for you see that God does not swear by another, for nothing is better than himself, but by himself, who is the best of all; but some have suggested as if it was inconvenient to swear, for an oath is taken for the sake of faith; but God alone is faithful''