“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” (Hebrews 6:17, NLT).
In the past, men (women didn’t usually need to do this, but that’s another story) made an oath when making a promise. It cemented the agreement and showed how serious the parties were. Now we sign contracts, which makes agreements legally binding. We have to do this due to the dishonesty or irresponsibility of so many. Gone are the days when someone’s word was all you needed to back up a promise.
When God made his famous promise to Abraham, he gave an oath. He didn’t really need to because he’s God—he’s perfect and can’t lie. But just to make sure Abraham and everyone else would know he was serious and would come through, he gave his word, which as The Message puts it, is a rock-solid guarantee. God is patient with us and his care and concern are marvelous. When we might say, how do we know you’ll keep your promise, instead of showing anger or frustration, he backs it up with an unbreakable pledge—his perfect word of honor.
Jesus - Son, Savior, God 