Charles Spurgeon Commentary Hebrews 7:15-18

"And [what we say] is yet more abundantly evident, if after the likeness of Melchizedek there ariseth another priest, who hath been made, not after the law of a carnal commandment, but after the power of an endless life: for it is witnessed [of him,] Thou art a priest for ever After the order of Melchizedek. For there is a disannulling of a foregoing commandment because of its weakness and unprofitableness" — Hebrews 7:15-18 (ASV)

The old Levitical law is annulled; it became weak and unprofitable; and now a higher and better dispensation is ushered in with a greater and undying priesthood.