An authorized mediator who offers a true sacrifice in acknowledgment of God's supreme dominion over human beings and in expiation for their sins. A priest's mediation is the reverse of that of a prophet, who communicates from God to the people. A priest mediates from the people to God.
Christ, who is God and man, is the first, last, and greatest priest of the New Law. He is the eternal high priest who offered himself once and for all on the Cross, a victim of infinite value, and he continually renews that sacrifice on the altar through the ministry of the Church.
Within the Church men who are specially ordained as priests to consecrate and offer the body and blood of Christ in the Mass. The Apostles were the first ordained priests, when on Holy Thursday night Christ told them to do in his memory what he had just done at the Last Supper. All priests and bishops trace their ordination to the Apostles. Their second essential priestly power, to forgive sins, was conferred by Christ on Easter Sunday, when he told the Apostles, "For those whose sins you forgive, they are forgiven; for those whose sins you retain, they are retained" (John 20-22,23).
All the Christian faithful, however, also share in the priesthood by their baptismal character. They are enabled to offer themselves in sacrifice with Christ through the Eucharistic liturgy. They offer the Mass in the sense that they internally unite themselves with the outward offering made by the ordained priest alone.