Hebrews 7:21-24

but with Jesus, God affirmed his royal-priesthood with his promise, saying, The Lord has made a solemn oath and will never change his mind, “You are a King-Priest forever!” So all of this magnifies the truth that we have a superior covenant with God than what they experienced, for Jesus himself is its guarantor! As additional proof, we know there were many priests under the old system, for they eventually died and their office had to be filled by another. But Jesus permanently holds his priestly office, since he lives forever and will never have a successor!