For when God made a promise to Abraham, since He had no one greater to swear by, He swore by Himself:
References for Hebrews 6:13
I will most certainly bless you, and I will greatly multiply you.
References for Hebrews 6:14
And so, after waiting patiently, Abraham obtained the promise.
References for Hebrews 6:15
For men swear by something greater than themselves, and for them a confirming oath ends every dispute.
Because God wanted to show His unchangeable purpose even more clearly to the heirs of the promise, He guaranteed it with an oath,
so that through two unchangeable things, in which it is impossible for God to lie, we who have fled for refuge might have strong encouragement to seize the hope set before us.
We have this [hope]-like a sure and firm anchor of the soul-that enters the inner sanctuary behind the curtain.
Jesus has entered there on our behalf as a forerunner, because He has become a "high priest forever in the order of Melchizedek."
References for Hebrews 6:20