When God gave Abraham his promise, he swore by himself since he couldn't swear by anyone greater.
References for Hebrews 6:13
He said, I will certainly bless you and multiply your descendants.
References for Hebrews 6:14
So Abraham obtained the promise by showing patience.
People pledge by something greater than themselves. A solemn pledge guarantees what they say and shuts down any argument.
When God wanted to further demonstrate to the heirs of the promise that his purpose doesn't change, he guaranteed it with a solemn pledge.
So these are two things that don't change, because it's impossible for God to lie. He did this so that we, who have taken refuge in him, can be encouraged to grasp the hope that is lying in front of us.
This hope, which is a safe and secure anchor for our whole being, enters the sanctuary behind the curtain.
That's where Jesus went in advance and entered for us, since he became a high priest according to the order of Melchizedek.