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Hebrews 6:17
Robinson-Pierpont Byzantine Textform · Berean Standard Bible
Ἐν ᾧ περισσότερον βουλόμενος ὁ θεὸς ἐπιδεῖξαι τοῖς κληρονόμοις τῆς ἐπαγγελίας τὸ ἀμετάθετον τῆς βουλῆς αὐτοῦ, ἐμεσίτευσεν ὅρκῳ,
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.