Charles Spurgeon Commentary Hebrews 5:6

"as he saith also in another [place,] Thou art a priest for ever After the order of Melchizedek." — Hebrews 5:6 (ASV)

As he says also in another place –

In Psalm 110 –

Thou art a priest for ever after the order of Melchisedec.

He does not assume the office by His own authority, but it is laid upon Him. He does not come as an amateur, but as an authorized priest of God.