Hebrews 7:11
11Now if perfection was through the Levitical priesthood (for under it the people have received the law), what further need was there for another priest to arise after the order of Melchizedek, and not be called after the order of Aaron?
If perfection had been by the Levitical priesthood—if it answered all God's designs and man's wants—why another priest after Melchizedek? Wesley: under it the people have received the law, whence some might infer perfection by that priesthood; from this single consideration priesthood and law must give way to a better priesthood and more excellent dispensation.
Levitical perfection was not attained; another order was needed. The note struck is "perfection" as what justifies and sanctifies—already pointing toward Wesley's later note that the law made nothing perfect. The Melchizedek priest is not optional decoration; Aaron's order must yield.
Calvin: no covenant between God and man is ratified except it rests on a priesthood; the Law was introduced under the Levitical priesthood. If the ministry under Aaron were perfect, nothing could be changed; it follows the Law's ministry was not perfect, for David's new order was introduced.
Common to both: the "if" proves imperfection. Priesthood and covenant stand together; another order means the old ministry was not the ultimate end. Perfection was never by Levi; the Son's order was always the goal the Psalm announced.