When God made a promise to Abraham, because he had no one greater by whom to swear, he swore by himself,
saying, "I will surely bless you and multiply you."
And thus Abraham, having patiently endured, obtained the promise.
References for Hebrews 6:15
Human beings, of course, swear by someone greater than themselves, and an oath given as confirmation puts an end to all dispute.
In the same way, when God desired to show even more clearly to the heirs of the promise the unchangeable character of his purpose, he guaranteed it by an oath,
so that through two unchangeable things, in which it is impossible that God would prove false, we who have taken refuge might be strongly encouraged to seize the hope set before us.
We have this hope, a sure and steadfast anchor of the soul, a hope that enters the inner shrine behind the curtain,
where Jesus, a forerunner on our behalf, has entered, having become a high priest forever according to the order of Melchizedek.