Hebrews 7:3, 15, 17, 21

He is without father or mother or genealogy, having neither beginning of days nor end of life, but resembling the Son of God he continues a priest forever.

This becomes even more evident when another priest arises in the likeness of Melchizedek

For it is witnessed of him, “You are a priest forever, after the order of Melchizedek.”

but this one was made a priest with an oath by the one who said to him: “The Lord has sworn and will not change his mind, ‘You are a priest forever.’”