G0276–ἀμετάθετος–ametathetos

In the same way, since it was God's desire to display more convincingly to the heirs of the promise how unchangeable His purpose was,

ἀμετάθετον ametatheton “unchangeableness”

He added an oath, in order that, through two unchangeable things, in which it is impossible for Him to prove false, we may possess mighty encouragement--we who, for safety, have hastened to lay hold of the hope set before us.

ἀμεταθέτων, ametathetōn “unchangeable,”