18 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.

Read Hebrews 6:18 Using Other Translations

That by two immutable things, in which it was impossible for God to lie, we might have a strong consolation, who have fled for refuge to lay hold upon the hope set before us:
so that by two unchangeable things, in which it is impossible for God to lie, we who have fled for refuge might have strong encouragement to hold fast to the hope set before us.
So God has given both his promise and his oath. These two things are unchangeable because it is impossible for God to lie. Therefore, we who have fled to him for refuge can have great confidence as we hold to the hope that lies before us.