Hebrews 7:21-24

but Christ became a priest with God’s oath. God said: “The Lord has made a promise and will not change his mind. ‘You are a priest forever.’ ” This means that Jesus is the guarantee of a better agreement from God to his people. When one of the other priests died, he could not continue being a priest. So there were many priests. But because Jesus lives forever, he will never stop serving as priest.