Hebrews 6:17

17 Because 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.

Read Hebrews 6:17 Using Other Translations

Wherein God, willing more abundantly to shew unto the heirs of promise the immutability of his counsel, confirmed it by an oath:
So when God desired to show more convincingly to the heirs of the promise the unchangeable character of his purpose, he guaranteed it with an oath,
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.