Hebrews 6:17

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

In this way God, being determined to show more abundantly to the heirs of the promise the immutability of his counsel, interposed with an oath;

Wherein God, being minded to show more abundantly unto the heirs of the promise the immutability of his counsel, interposed with an oath;

in which God, more abundantly willing to shew to the heirs of the promise the immutability of his counsel, did interpose by an oath,

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.