Albert Barnes Commentary Hebrews 7:17

"for it is witnessed [of him,] Thou art a priest for ever After the order of Melchizedek." — Hebrews 7:17 (ASV)

For he testifies. That this is the true account of it is proved by the testimony of God himself, that he was to be a Priest for ever. See Barnes on Hebrews 5:6.