For example, there was God's promise to Abraham. Since there was no one greater to swear by, God took an oath in his own name, saying:
"I will certainly bless you richly, and I will multiply your descendants into countless millions."
References for Hebrews 6:14
Then Abraham waited patiently, and he received what God had promised.
When people take an oath, they call on someone greater than themselves to hold them to it. And without any question that oath is binding.
God also bound himself with an oath, so that those who received the promise could be perfectly sure that he would never change his mind.
So God has given us 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 take new courage, for we can hold on to his promise with confidence.
This confidence is like a strong and trustworthy anchor for our souls. It leads us through the curtain of heaven into God's inner sanctuary.
Jesus has already gone in there for us. He has become our eternal High Priest in the line of Melchizedek.