NET
17 In the same way God wanted to demonstrate more clearly to the heirs of the promise that his purpose was unchangeable, and so he intervened with an oath,
Darby
17 Wherein God, willing to shew more abundantly to the heirs of the promise the unchangeableness of his purpose, intervened by an oath,
KJV
17 Wherein God, willing more abundantly to shew unto the heirs of promise the immutability of his counsel, confirmed [it] by an oath: