Hebrews 5:4-5
And no one takes on himself the honor [of head priest], but [he is appointed only] when he is called by God, as was the case with Aaron.
Hebrews 5:10
[So], God declared Him to be a Head Priest according to the order of Melchizedek.
Hebrews 7:11-14
So, if perfection [i.e., a right relationship with God] could have been attained through the Levitical priesthood, (for under it people had received the law of Moses), what further need would there have been for another priest to arise, according to the order of Melchizedek, who was not considered "according to the order of Aaron"?
Hebrews 7:23-28
And on one hand, many people were appointed to be [Levitical] priests because they were prevented by death from continuing [to serve]. [Note: The idea is that Levitical priests had to be constantly replaced].