Matthew Poole and continuators on Hebrews 6:13

13 Forwhen God made promise to Abraham , because he could swear by no greater , he sware by himself .

For is a confirmation by instance, that Faith and Patience had made some to inherit the Promises, as Abraham, and what was influencing of him in the exercising them, viz. God's Promise and Oath. God Almighty who was as able to perform as to make a Promise, Gen. 17.1. having made a Promise to Abraham the Father of Believers, that he would communicate some Temporal and Spiritual good, which by it he gave him a right to, and bound himself to perform, which summarily was Christ the Redeemer to be of his Seed, and Isaac his immediate Seed to be a Type of him; this Promise at the Offering up of his Son Isaac God confirms to him by Oath.

An Oath is to be made by the greatest, who is able to make good all, and to judge after his Will; by nothing under or beneath God must there be any Swearing.

As the best and greatest, Iehovah himself confirming that which was evident and certain by that which was most so: A strange condescension of God the Son, the Angel of the Covenant, to a Creature, to lift up his hand to Eternity, and to lay it on the Altar of his infinite and unchangeable Being, to pawn and pledge his Deity, that he might give the highest Assurance; and is willing that it shall be forfeited and lost, if Abraham fall short of what he hath promised to him, see the Oath, Gen. 22.15, to 19. An Oath confirming the Covenant of Grace to all Believers as firmly as to Abraham.