A distinction is to be made between "priest" and "presbyter." In the 1983 Code of Canon Law, "The Latin words sacerdos and sacerdotium are used to refer in general to the ministerial priesthood shared by bishops and presbyters. The words presbyter, presbyterium and presbyteratus refer to priests [in the English use of the word] and presbyters
While the consecrated life is neither clerical or lay by definition, clerics can be both members of institutes of consecrated life or secular (diocesan).
The theology of the Catholic priesthood is rooted in the priesthood of Christ and to some degree shares elements of the ancient Hebraic priesthood as well. A priest is one who presides over a sacrifice and offers that sacrifice and prayers to God on behalf of believers. The ancient Jewish priesthood which functioned at the temple in Jerusalem offered animal sacrifices at various times throughout the year for a variety of reasons.
In Christian theology, Jesus is the Lamb provided by God himself as a sacrifice for the sins of the world. Before his death on the cross, Jesus celebrated the Passover with his disciples and offered blessings over the bread (matzoh) and wine respectively, saying: "Take and eat. This is my body” and "Drink from this all of you, for this is my blood, the blood of the covenant, poured out for the forgiveness of sins." (Matthew 26:26b-28 Jerusalem Bible). The next day Christ's body and blood became visible in his sacrifice on the cross. Catholics believe that it is this same body, sacrificed on the cross and risen on the third day which is made present in the offering of each Eucharistic sacrifice which is called the Mass.
Thus priests (and bishops who are “high priests”) in presiding at the Eucharist join each offering of the bread and wine in union with the sacrifice of Christ. Catholic ordained ministers are known as priests because by their celebration of the Eucharist, they offer in a new moment in time the one eternal sacrifice of Christ.
Catholicism does not teach that Christ is sacrificed again and again, but that "The sacrifice of Christ and the sacrifice of the Eucharist are one single sacrifice.. Catholicism holds the Jewish concept of memorial in which "..the memorial is not merely a recollection of past events....these events become in a certain way present and real." and thus "...the sacrifice Christ offered once and for all on the cross remains ever present. Thus, Catholic clergy share in the one Priesthood of Christ.
After being called forward and presented to the assembly, the candidates are questioned. Each ordinand promises to diligently perform the duties of the Priesthood and to respect and obey his ordinary (bishop or religious superior). Then the candidates lie prostrate before the altar, while the assembled faithful kneel and pray for the help of all the saints in the Litany of the Saints.
The essential part of the rite is that when the bishop lays his hands in silence upon the each candidate (followed by all priests present), before offering the consecratory prayer, addressed to God the Father, invoking the power of the Holy Spirit upon the one being ordained.
After the consecratory prayer the newly ordained is vested with the stole and chasuble of those belonging to the Ministerial Priesthood and then the bishop anoints his hands with chrism and presents him with the holy chalice and paten which he will use when presiding at the Eucharist. Following this, the gifts of bread and wine are brought forward by the people and given to the new priest; then all the priests present, concelebrate the Eucharist with the bishop with the newly ordained taking the place of honour at the right of the bishop. If there are several newly ordained, it is they who gather closest to the bishop during the Eucharistic Prayer.
The laying of hands of the priesthood is found in 1 Timothy 4:14:
"Do not neglect the gift you have, which was conferred on you through the prophetic word with the imposition of hands of the presbyterate."Note that the word for presbyter or elder is from the same Greek word as priest. Imposition of hands or filling of hands, or semicha, in Hebrew was necessary for the installation of Joshua, rabbis, and Levite priests.
It is known that the First Ecumenical Council which took place at Nicaea included in its legislation a discipline of the priesthood known as clerical continence. This was the requirement of all priests and bishops to refrain from sexual contact with their wives or with any other woman; thus for a married man to become a priest, his wife had to agree to abstain from sexual relations. This discipline was reinforced in the legislation of various local councils such as the Council of Elvira, the date of which cannot be determined with exactness, but is believed to be in the first quarter of the fourth century, in Spain. It is evident that while priests were required to refrain from all sexual contact by virtue of their presiding at the Eucharist, this was an exceedingly difficult discipline to maintain. As the priests of the Jewish Temple in Jerusalem were required to abstain from sexual contact (in order to achieve ritual purity) for a brief period prior to the periodic performance of the sacrifices of the temple, so the priests of the Early Church were required by ecclesiastical law to abstain from sexual contact. However, they presided at the sacrifice of the Eucharist on a regular basis including every Sunday and the annual feasts of the various martyrs. Thus, they were not afforded by the Christian calendar periods in which they could be sexually active with their wives.
The Directa decretal was written by Pope Siricius in February 385 AD. It took the form of a long letter to Spanish bishop Himerius of Tarragona replying to the bishop’s requests on various subjects sent several months earlier to Pope Damasus I . It became the first of a series of documents published by the Magisterium that claimed apostolic origin for clerical celibacy and reminded ministers of the altar of the perpetual continence required of them.
===After the Great Schism=== Within a century of the schism of 1054 A.D., the Churches of the East and West came up with differing disciplines as alternatives to the very difficult practice of being married while abstaining from sexual contact. In the East, provision was made for candidates for the Priesthood to be married with permission to have a regular sexual relations with their wives. Still, a priest once ordained could not marry. Additionally, the Christian East did require that to become a bishop a priest must separate from his wife (wives are permitted to object), they becoming bishops and their wives typically becoming nuns. In the East, more normally, bishops are chosen from amongst those priests who are monks and thus unmarried.
In the West the law of celibacy was required by the 11th century. This law mandated that in order to become a candidate for ordination a man could not be married. The law remains in effect in the West, although not for those who are Eastern Rite Catholic clergy. The issue of mandatory celibacy continues to be a matter of some debate, though successive popes have declared that the discipline will not change.
Among the duties of a Catholic priest is the celebration of the Eucharist as a presider in persona Christi or as a concelebrant if required. The daily recitation of the principle and minor offices of the Liturgy of the Hours. Catholic priests are the only ministers of the Sacrament of Penance and Anointing of the Sick. They are the only ones who can celebrate the Eucharist in the Catholic Church (not to be confused with distribution by deacons or by extraordinary ministers). They and deacons are ordinary ministers of Baptism and witnesses to marriage.