Hebrews 7:15-17

A third proof that God made a change in the priesthood is that God predicted that Messiah would live forever (Psalms 110:4). Jesus Christ did not become a priest because He met a physical requirement, namely, was born into the priestly tribe and qualified by his descent to serve as high priest. He became a priest because He would not die. In this He showed Himself to be a member of Melchizedek’s "order" since Melchizedek appears from the scriptural record to have lived forever. Jesus is a priest forever because of His resurrection. [Note: See Manson, p. 116.]