Hebrews 6:13-19

For when God made a promise to Abraham, since he had no one greater to swear by, he swore by himself: I will indeed bless you, and I will greatly multiply you. And so, after waiting patiently, Abraham obtained the promise. For people 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 as an anchor for the soul, firm and secure. It enters the inner sanctuary behind the curtain.