(For those priests were made without an oath; but this with an oath by him that said unto him, The Lord sware and will not repent, Thou art a priest for ever after the order of Melchisedec:)
Greek order, 'For they indeed (the existing legal priests) without the (solemn) promise on oath [horkomosias] are priests made' (Tittmann).
By him - by God.
Unto him - the Lord, the Son of God (Psalms 110:1).
Not repent - never change His purpose.
After the order of Melchisedec - So A Delta f. Omitted in 'Aleph (') B C, Vulgate.