Hebrews 6:13-20 NIV
When God made his promise to Abraham, since there was no one greater for him to swear by, he swore by himself, saying, “I will surely bless you and give you many descendants.” And so after waiting patiently, Abraham received what was promised.
People swear by someone greater than themselves, and the oath confirms what is said and puts an end to all argument. Because God wanted to make the unchanging nature of his purpose very clear to the heirs of what was promised, he confirmed it with an oath. God did this so that, by two unchangeable things in which it is impossible for God to lie, we who have fled to take hold of the hope set before us may be greatly encouraged. We have this hope as an anchor for the soul, firm and secure. It enters the inner sanctuary behind the curtain, where our forerunner, Jesus, has entered on our behalf. He has become a high priest forever, in the order of Melchizedek.
NIV: New International Version