"Holy Orders is the sacrament through which the mission entrusted by Christ to his apostles continues to be exercised in the Church until the end of time…It includes three degrees of order: episcopate, presbyterate, and diaconate" (CCC 1536). Deacons, priest and bishops are essential to the Catholic Church because we believe that they continue the work begun by the apostles.
Since the beginning, the ordained ministry has been conferred and exercised in three degrees: that of bishops, that of presbyters, and that of deacons. The ministries conferred by ordination are irreplaceable for the organic structure of the Church: without the bishop, presbyters, and deacons, one cannot speak of the Church. (CCC 1593)