When God made his promise to Abraham, he took an oath to keep it. But there was no one greater than himself to take an oath by. So he took his oath by making an appeal to himself.
References for Hebrews 6:13
He said, "I will certainly bless you. I will give you many children."(Genesis 22:17)
Abraham was patient while he waited. Then he received what God promised him.
People take oaths by someone greater than themselves. An oath makes a promise certain. It puts an end to all arguing.
So God took an oath when he made his promise. He wanted to make it very clear that his purpose does not change. He wanted those who would receive what was promised to know that.
God took an oath so we would have good reason not to give up. We have run away from everything else to take hold of the hope offered to us in God's promise. So God gave his promise and his oath. Those two things can't change. He couldn't lie about them.
Our hope is certain. It is something for the soul to hold on to. It is strong and secure. It goes all the way into the Most Holy Room behind the curtain.
That is where Jesus has gone. He went there to open the way ahead of us. He has become a high priest forever, just like Melchizedek.