With the adjective that is translated “unalterable”
(aparabatos), the author underscores the decisive nature of the change
that has taken place. This term is attested in papyri, where it is found in
business contracts, stipulating that the terms of a contract can never be
changed (M[oulton]M[illigan]). Whereas the Levitical priests kept being
replaced and the legislation on which they based their existence was overturned
and abolished (vv. 12, 18). Jesus has a priesthood that never changes. (Sigurd
Grindheim, The Letter to the Hebrews [The Pillar New Testament
Commentary; Grand Rapids, Mich.: Eerdmans, 2023], 368)