What is the significance of the phrase "Thou art a priest for ever after the order of Melchisedec" which occurs three times in the Epistle to the Hebrews?

The Office of Christ (Heb 5:6)
"Thou art a priest for ever after the order of Melchisedec"

The Order of Perfection (Heb 7:17)
"Thou art a priest for ever after the order of Melchisedec"

The Oath of God (Heb 7:21)
"Thou art a priest for ever after the order of Melchisedec"