Hebrews 6:16-19

For men swear by one greater than themselves, and with them an oath given as confirmation is an end of every dispute. In the same way God, desiring even more to show to the heirs of the promise the unchangeableness of His purpose, interposed with an oath, so that by two unchangeable things in which it is impossible for God to lie, we who have taken refuge would have strong encouragement to take hold of the hope set before us. This hope we have as an anchor of the soul, a hope both sure and steadfast and one which enters within the veil