This Volume III has only one goal: to help students understand the coming of Jesus, the Son of God, in our world to cohabitate with us. The priesthood of Aaron, as willed by God, did not have the divine power to forgive sins. Thus, God, in his second plan, as saint Paul reveals, to offer to us his salvation, sent us His Son consecrated the Eternal High Priest in the very first moment of his Incarnation. God does not oblige anyone to accept salvation that He offers us in Jesus Eternal High Priest. However, the priesthood of Christ is absolutely necessary to work our salvation. This Volume III puts in clear terms, acceptable to the people, the essential doctrine of the Priesthood of Christ in our salvation.