ISB ~ Hebrews 5 v1 - 4

Priests are mediators between man and God. As such, they must be united with men and called by God. The priests of Israel, who traced their lineage to Aaron and his sons, were united with men in the weakness of human sin and were called by God to an earthly priesthood (Ex 28:1-4; 40:12-15). Jesus, too, was united with men in his human nature (Heb 2:14); however, unlike the Aaronic priests, he was free from sin (4:15) and was called by God to a heavenly priesthood (5:5-6; 8:14). Priestly ministry is a vocation from God, not volunteer work that men can take upon themselves (CCC 1539, 1578).