Hebrews 6:16-18

16For people swear by something greater than themselves, and for them a confirming oath ends every dispute.

17Because God wanted to show his unchangeable purpose even more clearly to the heirs of the promise, he guaranteed it with an oath,

18so 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.