Hebrews 6:13-17

13When God made his promise to Abraham, since there was no one greater for him to swear by, he swore by himself, 14saying, “I will surely bless you and give you many descendants.” 15And so after waiting patiently, Abraham received what was promised. 16People swear by someone greater than themselves, and the oath confirms what is said and puts an end to all argument. 17Because God wanted to make the unchanging nature of his purpose very clear to the heirs of what was promised, he confirmed it with an oath.