Hebrews 6:13-17

God made a promise to Abraham. And as there is no one greater than God, he used himself when he swore to Abraham, saying, “I will surely bless you and give you many descendants.” Abraham waited patiently for this to happen, and he received what God promised. People always use the name of someone greater than themselves when they swear. The oath proves that what they say is true, and this ends all arguing. God wanted to prove that his promise was true to those who would get what he promised. And he wanted them to understand clearly that his purposes never change, so he made an oath.