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.
Textus Receptus (TR) - Hebrews 6:17 >Ἐν ᾧ περισσότερον βουλόμενος ὁ Θεὸς ἐπιδεῖξαι τοῖς κληρονόμοις τῆς ἐπαγγελίας τὸ ἀμετάθετον τῆς βουλῆς αὐτοῦ, ἐμεσίτευσεν ὅρκῳ,