Hebrews 6:16-17

Twentieth_Century(i) 16 Men, of course, swear by what is greater than themselves, and with them an oath is accepted as putting a matter beyond all dispute. 17 And therefore God, in his desire to show, with unmistakable plainness, to those who were to enter on the enjoyment of what he had promised, the unchangeableness of his purpose, bound himself with an oath.