Hebrews 7:3, 15, 17, 21

Without father or mother, without genealogy, without beginning of days or end of life, resembling the Son of God, he remains a priest forever.

And what we have said is even more clear if another priest like Melchizedek appears

For it is declared: “You are a priest forever, in the order of Melchizedek.”

but he became a priest with an oath when God said to him: “The Lord has sworn and will not change his mind: ‘You are a priest forever.’ ”