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 offered to 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 Jesus, who went before us, has entered on our behalf. He has become a high priest forever, in the order of
Melchizedek.
Hebrews 6:17-20
(NIV)