In this matter, God, wanting to reveal more thoroughly the immutability of his counsel to the heirs of the promise, interposed an oath
Other versions
King James Version
Wherein God, willing more abundantly to shew unto the heirs of promise the immutability of his counsel, confirmed it by an oath: English Standard Version
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,New International Version
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.