When a man takes an oath, he is calling upon someone greater than himself to force him to do what he has promised or to punish him if he later refuses to do it; the oath ends all argument about it.
God also bound Himself with an oath, so that those he promised to help would be perfectly sure and never need to wonder whether He might change His plans.
He has given us both His Promise and His Oath, two things we can completely count on, for it is impossible for God to tell a lie. Now all those who flee to Him to save them can take new courage when they hear such assurances from God; now they can know without doubt that He will give them the salvation He has promised them.
Hebrews 6:16-18 (TLB)