Holy Orders is the sacrament through which the mission of Jesus (to proclaim the Gospel) continues to be carried out in the Church. It is the sacrament through which the ministry of the Church is conferred. It includes three degrees: episcopate (bishop), presbyterate (priest), and deaconate (deacon). Christ is himself the source of ministry in the Church. He instituted the Church. He gave her authority and a mission. He set up a variety of offices aimed at the good of the whole Church.
It is through this sacrament that men are given power and authority and sent: to proclaim the Gospel, to speak in the name of Christ, to act in the person of Christ (persona Christi Capitis). Christ is present to the Church through the service of the ordained minister. Christ is made visible in the midst of the community of believers. They are authorized and empowered by Christ himself through the successors of the apostles.
Only a baptized man validly receives sacred ordination. The Lord Jesus chose men to form the college of the twelve apostles, and the apostles did the same when they chose collaborators to succeed them in their ministry. The college of bishops, with whom the priests are united in the priesthood, makes the college of the twelve an ever-present and ever-active reality until Christ's return. The Church recognizes herself to be bound by this choice made by the Lord himself. For this reason the ordination of women is not possible. (CCC 1577)
All the ordained ministers of the Latin Church, with the exception of permanent deacons, are normally chosen from among men of faith who live a celibate life and who intend to remain celibate "for the sake of the kingdom of heaven." Called to consecrate themselves with undivided heart to the Lord and to "the affairs of the Lord," they give themselves entirely to God and to men. Celibacy is a sign of this new life to the service of which the Church's minister is consecrated; accepted with a joyous heart celibacy radiantly proclaims the Reign of God. (CCC 1579)
In the Eastern Churches a different discipline has been in force for many centuries: while bishops are chosen solely from among celibates, married men can be ordained as deacons and priests. This practice has long been considered legitimate; these priests exercise a fruitful ministry within their communities. Moreover, priestly celibacy is held in great honor in the Eastern Churches and many priests have freely chosen it for the sake of the Kingdom of God. In the East as in the West a man who has already received the sacrament of Holy Orders can no longer marry. (CCC 1580)