Hebrews 5:4-6

To be a high priest is an honor, but no one chooses himself for this work. He must be called by God as Aaron was. So also Christ did not choose himself to have the honor of being a high priest, but God chose him. God said to him, “You are my Son. Today I have become your Father.” And in another Scripture God says, “You are a priest forever, a priest like Melchizedek.”