In the liturgical traditions the Catholic Church the term ordination refers to the means by which a person is included in one of the orders of bishops, priests or deacons. The teaching of the Catholic Church on ordination, as expressed in the Code of Canon Law, the Catechism of the Catholic Church, and the apostolic letter Ordinatio sacerdotalis, is that only a Catholic male validly receives ordination, and "that the Church has no authority whatsoever to confer priestly ordination on women and that this judgment is to be definitively held by all the Church's faithful." As with priests and bishops, the church ordains only men as deacons. The church cannot ordain anyone who has undergone sex reassignment surgery, and may sanction or require therapy for priests who are transsexual, regarding these to be an indicator of mental instability.
References are made within the earliest Christian communities to the ordination of men (through the laying on of hands), and there are several references to the role of women. Paul's letter to the Romans, written in the first century AD, for example: "I commend to you our sister Phoebe, a deaconess of the church at Cen'chre-ae, that you may receive her in the Lord as befits the saints, and help her in whatever she may require from you, for she has been a helper of many and of myself as well."
In AD 494, in response to reports that women were serving at the altar in the south of Italy, Pope Gelasius I wrote a letter condemning female participation in the celebration of the Eucharist, teaching that those roles were reserved for men.
Many Church Fathers did not advocate for or permit the ordination of women.Clement of Rome taught that the apostles chose only men to succeed them, The First Council of Nicaea, the first ecumenical council, subsequently decreed that deaconesses were not ordained ministers because they did not receive the laying on of hands, were to be considered lay persons.
Concerning the "constant practice of the Church", in antiquity the Church Fathers Irenaeus,Tertullian,Hippolytus,Epiphanius,John Chrysostom, and Augustine all wrote that the priestly ordination of women was impossible. The Council of Laodicea prohibited ordaining women to the Presbyterate, although the meaning of Canon 11 has received very different interpretations as to whether it refers to senior deaconesses or older women presiding over the female portion of the congregation. In the period between the Reformation and the Second Vatican Council, mainstream theologians continued to oppose the priestly ordination of women, appealing to a mixture of scripture, Church tradition and natural law.[Note 1] Even so, mainstream theologians did not dismiss the ordination of women as deacons.
Ecumenical councils, according to the church, are apart of the universal and extraordinary magisterium, making their canons and decrees infallible insofar as they are about the Catholic faith and morals. Canon 19 of the First Council of Nicaea (325 CE) declared deaconesses to be laywomen:
Like treatment should be given in the case of their deaconesses, and generally in the case of those who have been enrolled among their clergy. We mean by 'deaconesses' those who have assumed the habit, but who, since they have not had hands laid upon them, are to be numbered only among the laity.
No woman under forty years of age is to be ordained a deacon, and then only after close scrutiny. If after receiving ordination and spending some time in the ministry she despises God's grace and gets married, such a person is to be anathematised along with her spouse.
While the church believes Christians have the right to receive the sacraments, the church does not believe in a right to ordination. The church believes the sacraments work ex opere operato as manifestations of Jesus' actions and words during his life, and that according to dogma Jesus only chose certain men as apostles. The church teaches that a woman's impediment to ordination is diriment, of divine law, public, absolute, and permanent because Jesus instituted ordination by ordaining the twelve apostles, since holy orders is a manifestation of Jesus' calling of the apostles.
In 1976, the Congregation for the Doctrine of the Faith issued the Declaration on the Question of the Admission of Women to the Ministerial Priesthood which taught that for doctrinal, theological, and historical reasons, the Church "does not consider herself authorized to admit women to priestly ordination". Reasons given were the Church's determination to remain faithful to its constant tradition, its fidelity to Christ's will, and the iconic value of male representation due to the "sacramental nature" of the priesthood. The Church teaching on the restriction of its ordination to men is that masculinity was integral to the personhood of both Jesus and the men he called as apostles. The Church sees maleness and femaleness as two different ways of expressing common humanity (essence).
Wherefore, in order that all doubt may be removed regarding a matter of great importance, a matter which pertains to the Church's divine constitution itself, in virtue of my ministry of confirming the brethren (cf. Lk 22:32) I declare that the Church has no authority whatsoever to confer priestly ordination on women and that this judgment is to be definitively held by all the Church's faithful.
Pope John Paul II explains the Catholic understanding that the priesthood is a role specially set out by Jesus when he called twelve men out of his group of male and female followers. John Paul says that Jesus chose the Twelve after a night in prayer and that the Apostles themselves were careful in the choice of their successors. The priesthood is "specifically and intimately associated in the mission of the Incarnate Word himself
Pope Paul VI, quoted by John Paul in Ordinatio sacerdotalis, wrote, "The Church holds that it is not admissible to ordain women to the priesthood, for very fundamental reasons. These reasons include: the example recorded in the Sacred Scriptures of Christ choosing his Apostles only from among men; the constant practice of the Church, which has imitated Christ in choosing only men; and her living teaching authority which has consistently held that the exclusion of women from the priesthood is in accordance with God's plan for his Church."
On Oct 28, 1995, in response to a dubium concerning Ordinatio sacerdotalis, the Congregation for the Doctrine of the Faith said:
On July 15, 1998 the Congregation for the Doctrine of the Faith issued a doctrinal commentary on Ad tuedam fidem. In it, the Congregation gave examples of Catholic doctrines owed the full assent of faith, including the reservation of ordination to men only:
A similar process can be observed in the more recent teaching regarding the doctrine that priestly ordination is reserved only to men. The Supreme Pontiff, while not wishing to proceed to a dogmatic definition, intended to reaffirm that this doctrine is to be held definitively, since, founded on the written Word of God, constantly preserved and applied in the Tradition of the Church, it has been set forth infallibly by the ordinary and universal Magisterium. As the prior example illustrates, this does not foreclose the possibility that, in the future, the consciousness of the Church might progress to the point where this teaching could be defined as a doctrine to be believed as divinely revealed.
On December 2, 2002, the Congregation for the Doctrine of the Faith issued the Decree on the Attempted Ordination of Some Catholic Women. In it, the Congregation states that the doctrine of ordination was definitively proposed by John Paul II in Ordinatio sacerdotalis:
In addition there is the doctrinal aspect, namely, that they formally and obstinately reject a doctrine which the church has always taught and lived, and which was definitively proposed by Pope John Paul II, namely, 'that the church has no authority whatsoever to confer priestly ordination on women' (Ordinatio sacerdotalis, 4). The denial of this doctrine is rightly considered the denial of a truth that pertains to the Catholic faith and therefore deserves a just penalty (cf. Canons 750 §2; 1372, n. 1; John Paul II, Ad Tuendam Fidem, 4A).
The Congregation further stated that to deny the dogma is to oppose the magisterium of the Pope:
Moreover, by denying this doctrine, the persons in question maintain that the magisterium of the Roman Pontiff would be binding only if it were based on a decision of the college of bishops, supported by the sensus fidelium and received by the major theologians. In such a way they are at odds with the doctrine on the magisterium of the successor of Peter, put forward by both the First and Second Vatican Councils, and they thereby fail to recognize that the teachings of the supreme pontiff on doctrines to be held definitively by all the faithful are irreformable.
The Lord Jesus chose men (viri) to form the college of the twelve apostles...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.
The Vatican's Congregation for the Doctrine of the Faith issued and published on May 29, 2008, in the Vatican newspaper L'Osservatore Romano, a decree signed by Cardinal William Levada, by determining that women "priests" and the bishops who attempt to ordain them would incur excommunication latae sententiae.
Pope Francis said "that door is closed" regarding women's priestly ordination, affirming the teachings of his predecessors, including Saint Pope John Paul II and Pope Emeritus Benedict XVI. He expanded on this in a November 2016 informal statement on the return flight from his papal visit to Sweden to commemorate the Reformation: "On the ordination of women in the Catholic Church, the final word is clear, it was said by St. John Paul II and this remains." Francis added that women are very important to the Church, specifically from a "Marian dimension. In Catholic ecclesiology there are two dimensions to think about ... The Petrine dimension, which is from the Apostle Peter, and the Apostolic College, which is the pastoral activity of the bishops, as well as the Marian dimension, which is the feminine dimension of the Church." The Church is depicted as the bride of Christ, as a woman.
The ordination of females to the diaconate is being discussed by many scholars, including Roman Catholic historians and theologians, as well as Pope Francis.
Further historical evidence points to women serving in many areas of the Church in the West as well as in the East. Monastic female deacons in the East received the stole as a symbol of their office at ordination, which took place inside the sanctuary. Historical-theological work by K. K. Fitzgerald, Phyllis Zagano, and Gary Macy argue for the sacramental ordination of women as deacons. A significant contribution on this aspect was made by a future cardinal, Jean Daniélou, in an article in La Maison-Dieu in 1960 (translated into English as "The Ministry of Women in the Early Church" by Glyn Simon, Anglican Bishop of Llandaff).
The Congregation for the Doctrine of the Faith wrote in 1977 that the possibility of ordaining women as deacons was "a question that must be taken up fully by direct study of the texts, without preconceived ideas." The opinion that women received sacramental ordination (in certain times and places) is given by Roger Gryson. In response, Aimé Georges Martimort contends they did not. Both Gryson and Martimort argue from the same historical evidence, and Martimort agrees the matter is not settled. For example, the ecumenical First Council of Nicaea (325) stated that deaconesses of heretical sects "do not receive any imposition of hands, so that they are in all respects to be numbered among the laity." However, 126 years later, the ecumenical Council of Chalcedon (451) decreed "A woman shall not receive the laying on of hands as a deaconess under forty years of age, and then only after searching examination." Gryson argues that the use of the verb cheirotonein and of the substantive cheirothesia clearly indicate that women deacons were ordained by the laying on of hands. Martimort argues that the "laying on of hands" refers only to a special blessing, although cheirotonein is clearly used with an invocation of the Holy Spirit.
Women ceased to function as deacons in the West in the 13th century, although female deacons are still ordained in some churches of Orthodoxy. The Second Vatican Council revived the permanent diaconate, raising the question from a purely theoretical matter to one with immensely practical consequences. Based on the hypothesis that women deacons received and are capable of receiving the sacrament of Holy Orders, there have been continued modern-day proposals to ordain female permanent deacons, who would perform the same functions as male deacons and be like them in every respect.
The International Theological Commission worked on a document between 1992-1997; between 1997-2002 a new configuration of the International Theological Commission again studied the question and its report was approved for publication by the then-Prefect of the Congregation for the Doctrine of the Faith, Cardinal Joseph Ratzinger. The 2002 document states that the matter is one for the Magisterium to decide.
In 2003, Father Ronald G. Roberson, a staff member of the United States Conference of Catholic Bishops, stated in a presentation on the diaconate in the Latin Church to annual meeting of the U.S. Oriental Orthodox-Roman Catholic Consultation: "The possibility of ordaining women to the diaconate is still an unsettled question in the Catholic Church. Latin rituals for ordaining deaconesses exist from as late as the 10th century, but the precise sacramental nature of these ordinations has not yet been determined authoritatively. There are recent indications that the Holy See intends to continue the exclusion of women from this office."
In October, 2015, Archbishop Jean-Paul Durocher, of Gatineau, Quebec, called for the restoration of women to the diaconate at the Synod of Bishops on the Family.
In August 2016, Pope Francis created a commission to study the ministry of deaconesses in the early church, exploring their roles, the rites they participated in, and the formulas employed to designate them as deaconesses. The Commission included twelve scholars under the presidency of Cardinal Luis Ladaria Ferrer, Prefect of the International Theological Commission. The first Commission meetings were held November 25-26 in Rome. In January 2019, two members of the Pontifical Commission for the Study of the Diaconate of Women said that a report had been submitted to Pope Francis from the Commission. On June 20, 2018 Francis said that there had been no conclusive decisions, but that he was not afraid of ongoing studies.
In October 2019, Members of the Synod of Bishops for the Amazon asked that women be given leadership roles in the Catholic Church, although they stopped short of calling for female deacons., but there were many bishops also, who voted by 137 to 30  in favour of female deacons.
In Mulieris dignitatem, Pope John Paul II advocated for Christian complementarianism, writing: "In calling only men as his Apostles, Christ acted in a completely free and sovereign manner. In doing so, he exercised the same freedom with which, in all his behavior, he emphasized the dignity and the vocation of women, without conforming to the prevailing customs and to the traditions sanctioned by the legislation of the time."
In Ordinatio sacerdotalis, John Paul II wrote: "the fact that the Blessed Virgin Mary, Mother of God and Mother of the Church, received neither the mission proper to the Apostles nor the ministerial priesthood clearly shows that the non-admission of women to priestly ordination cannot mean that women are of lesser dignity, nor can it be construed as discrimination against them. Rather, it is to be seen as the faithful observance of a plan to be ascribed to the wisdom of the Lord of the universe."
The Roman Catholic Church does not regard the priest as the only possible prayer leader, and prayer may be led by a woman. For example, when no priest, deacon, instituted lector or instituted acolyte is available, lay people (either men and women) may be appointed by the pastor to celebrate a Liturgy of the Word and distribute Holy Communion (which must be consecrated beforehand by a priest). During these liturgies, a layperson is to act as "one among equals" and avoid formulas or rites which are proper to ordained ministers.
Religious life is a distinct vocation in itself, and women live in consecrated life as a nun or religious sister, and throughout the history of the Church it has not been uncommon for an abbess to head a dual monastery, i.e., a community of men and women. Women today exercise many roles in the Church. They run catechetical programs in parishes, provide spiritual direction, serve as lectors and extraordinary ministers of Holy Communion, and teach theology. In 1994, the Congregation for Divine Worship and the Discipline of the Sacraments interpreted the 1983 Code of Canon Law to permit girls and women to assist at Mass as altar servers. Still many people see the Church's position on the ordination of women as a sign that women are not equal to men in the Catholic Church, though the Church rejects this inference.
Since 2002 Roman Catholic Womenpriests has conducted ordination ceremonies for women to become deacons, priests and bishops, saying that these ordinations are valid because the initial ordinations were conferred by a validly ordained Catholic male bishop (Romulo Antonio Braschi, who left the Roman Catholic Church in 1975) and therefore they are in the line of apostolic succession. However, the Catholic Church considers these ordinations to be invalid, and decreed excommunications for those involved in the ceremonies. Roman Catholic Womenpriests interprets the works of certain Catholic scholars (for example, former minister John Wijngaards, liturgical reformist Robert W. Hovda, and theologian Damien Casey), to say that they have doctrinal support for the ordination of women.
Women's Ordination Worldwide, founded in 1996 in Austria, is a network of twelve national and international groups whose primary mission is the admission of Roman Catholic women to all ordained ministries. Members include Catholic Women's Ordination (founded in March 1993 in the United Kingdom), Roman Catholic Womenpriests (founded in 2002 in America), Women's Ordination Conference (founded in 1975 in America) and others. The first recorded Catholic organization advocating for women's ordination was St. Joan's Alliance, founded in 1911 in London. In February 2011, 144 German-speaking academic theologians (making up one-third of the Catholic theology professors in Germany, Austria, and Switzerland) submitted a document styled as Church 2011 calling for a list of concessions including "women in (the) ordained ministry." 
In 2014 the Bishop of Basel, Felix Gmür, allowed the Basel Catholic church corporations, which are officially only responsible for church finances, to formulate an initiative appealing for equality between men and women in ordination to the priesthood. Also in 2014, the Association of Catholic Priests in Ireland stated that the Catholic Church must ordain women and allow priests to marry in order to survive.
Other notable dissenters on the subject of women's ordination are the Austrian-based Call to Disobedience, also as of 2013 a minority in the American "Association of U.S. Catholic Priests" favor ordaining women as priests, and the majority of that organisation favour allowing woman deacons.
Catholic women religious were major participants in the first and second meetings of the Women's Ordination Conference. In 1979, Sister Theresa Kane, then the president of the Leadership Conference of Women Religious, spoke from the podium at Washington, DC's Shrine of the Immaculate Conception, and asked Pope John Paul II to include women "in all ministries of our Church." In the audience were nearly fifty sisters wearing blue armbands, symbolizing women's ordination.
When Pew Research polled Americans in 2015, 59 percent of those who self-identified as Catholic believed that the church should ordain women.
A "History of the women's ordination movement in the U.S. Roman Catholic church" was published in 2015.
There is at least one organization that calls itself "Roman Catholic" that ordains women as priests at the present time, Roman Catholic Womenpriests; and several independent Catholic jurisdictions have been ordaining women in the United States since approximately the late 1990s. These organizations are independent of, and unrecognised by, the Roman Catholic Church. There are several others calling for the Roman Catholic Church itself to ordain women, such as St. Joan's International Alliance, Circles, Brothers and Sisters in Christ, Catholic Women's Ordination, and Corpus, along with others. On April 19, 2009, Womenpriests elected four bishops to serve the United States: Joan Mary Clark Houk, Andrea Michele Johnson, Maria Regina Nicolosi, and Bridget Mary Meehan. The Vatican's Congregation for the Doctrine of the Faith had issued a decree in 2008 declaring such "attempted ordinations" invalid and that, since Canons 1378 and 1443 apply to those who participate in these ceremonies, all were excommunicated. Edward Peters, a doctor of canon law, explains that their excommunication results in virtue of a combination of other canons which arise from application of Canons 1378 and 1443. In response, Womenpriests said its members are "loyal member of the church who stand in the prophetic tradition of holy disobedience to an unjust law."
In 2017, German bishop Gebhard Fürst supported the ordination of women to the diaconate. In October 2019 German bishop Franz-Josef Overbeck said many Catholic people don't understand why women are unable to be deacons or priests, which he thinks should be changed.
At the Synod on the Amazon in Rome in October 2019 many bishops said that women should be allowed to become deacons.