A.T. Robertson Commentary Hebrews 7:24

"but he, because he abideth for ever, hath his priesthood unchangeable." — Hebrews 7:24 (ASV)

Because he abideth (δια το μενειν αυτον). Same idiom as in verse 23, "because of the abiding as to him" (accusative of general reference, αυτον).

Unchangeable (απαραβατον). Predicate adjective in the accusative (feminine of compound adjective like masculine), late double compound verbal adjective in Plutarch and papyri, from alpha privative and παραβαινω, valid or inviolate. The same idea in verse 3. God placed Christ in this priesthood and no one else can step into it. See verse 11 for ιερωσυνη.