King James Version (KJV) - Hebrews 6:17 >

Where in God, willing more abundantly to shew unto the heirs of promise the immutability of his counsel, confirmed [it] by an oath:

Berean Study Bible (BSB) - Hebrews 6:17 >

So when God wanted to make the unchanging nature of His purpose very clear to the heirs of the promise, He guaranteed it with an oath.