Mystici corporis Christi (29 June 1943) is a papal encyclical issued by Pope Pius XII during World War II, on the Church as the Mystical Body of Christ. It is one of the more important encyclicals of Pope Pius XII, because of its topic, the Church, and because its Church concept was fully included in Lumen gentium but also strongly debated during and after Vatican II. The Church is called body, because it is a living entity; it is called the body of Christ, because Christ is its Head and Founder; it is called mystical body, because it is neither a purely physical nor a purely spiritual unity, but supernatural.
The encyclical followed the commencement of Nazi Germany's programs of "euthanasia" of the disabled, and race-based murders of Jews and other minorities, and is therefore significant for its reiteration of Church teachings against racism and the killings of people with disabilities.
The encyclical builds on a theological development in the 1920s and 1930s in Italy, France, Germany and England, which all re-discovered the ancient Pauline concept of the Mystical Body of Christ. Pius XII utilized these new discoveries and authoritatively added his directions to them, as the Dutch Jesuit Sebastian Tromp documented. Reginald Garrigou-Lagrange, Raimondo Spiazzi and Mariano Cordovani, all professors at the Pontificium Athenaeum Internationale Angelicum, the future Dominican Order-affiliated Pontifical University of Saint Thomas Aquinas, had a great influence on the drafting of the encyclical.
According to the encyclical, the Church has two aspects: one visible and one invisible. Those who live under the visible representative of Christ have full membership. Further, the relationship of the faithful and Christ is mystical, not physical. The faithful, through their faith, hope, and love, are united with Christ in the Church. Christ loves and lives in the faithful. Christ and the Church as the whole Church is made alive by the Holy Spirit, which also guides each of the faithful, and as such is an important element of the body of Christ. The unification with Christ takes place in the Holy Eucharist. Within the Church, there does not exist a bifurcated active and passive element (e.g. leadership and lay people), but rather all members of the Church are called to work on the perfection of the body of Christ.
The encyclical teaches, that while lay people animate human society, the Successors of the Apostles (the Catholic Bishops) are to be responsible in matters of religion and morals. Until this encyclical of Pius XII, Church was considered as societas perfecta, a perfect society, consisting primarily of Pope, bishops, clergy and the religious. Mystici Corporis includes lay people as equal and important elements of the body of Christ. The faithful are united with Christ in the Church. Christ loves and lives in them. Christ is alive through the Holy Spirit.
The encyclical states that Christ, while still on earth, instructed by precept, counsel and warnings "in words that shall never pass away, and will be spirit and life" to all men of all times. He conferred a triple power on His Apostles and their successors, to teach, to govern, to lead men to holiness, making this power, defined by special ordinances, rights and obligations, the fundamental law of the whole Church. God governs directly and guides personally the Church which He founded. Pius quoted Proverbs 21:1 noting that God reigns within the minds and hearts of men, and bends and subjects their wills to His good pleasure, even when rebellious.
Mystici corporis requests the faithful to love their Church and to always see Christ in her, especially in the old and sick members. They must accustom themselves to see Christ Himself in the Church. For it is Christ who lives in His Church, and through her, teaches, governs, and sanctifies; it is Christ also who manifests Himself differently in different members of His society.
If the faithful strive to live in a spirit of lively faith, they will not only pay due honor and reverence "to the more exalted members" of this Mystical Body, especially those who according to Christ’s mandate will have to render an account of our souls, but they will take to their hearts those members who are the object of our Savior’s special love: the weak, the wounded, and the sick who are in need of material or spiritual assistance; children whose innocence is so easily exposed to danger in these days; and finally the poor, in helping whom is recognized the very person of Jesus Himself as a perfect model of love for the Church.
In brief, this document which sees the Church as the one and only means of salvation, has been challenged by the Documents of Vatican II, especially Lumen Gentium, where we read in 1,8, that the true Church of Christ subsists in this church, but elements can also be found in other Christian Churches. As a result, the view of the Roman Catholic Church as the ONLY means of salvation has been challenged and people seem to more and more identify with the latter rather than the former.
Pope Pius XII condemns forced conversions in strong terms. These had been opposed by previous Popes such as Leo XIII, and are in violation of existing Canon Law, the law of the Church. Church membership and conversions must be voluntary. Regarding conversions, "We recognize that this must be done of their own free will; for no one believes unless he wills to believe." Hence they are most certainly not genuine Christians who against their belief are forced to go into a church, to approach the altar and to receive the Sacraments; for the "faith without which it is impossible to please God"  is an entirely free "submission of intellect and will."
The encyclical concludes with a summary of the mariology of the Pope. The 1854 dogma of the Immaculate Conception by Pius IX defined the Virgin conceived without sin, as the mother of God and our mother. Pope Pius XII built on this in Mystici corporis: Mary, whose sinless soul was filled with the divine spirit of Jesus Christ above all other created souls, "in the name of the whole human race" gave her consent "for a spiritual marriage between the Son of God and human nature", thus elevating human nature beyond the realm of the purely material. She who, according to the flesh, was the mother of our Head, became mother of all His members. Through her powerful prayers, she obtained that the spirit of our Divine Redeemer, should be bestowed on the newly founded Church at Pentecost.
While the Early Fathers of the Church tended to contrast Eve's disobedience with Mary's fiat at the Annunciation, Pius looked rather to her presence at Calvary where "...she, the second Eve, who, free from all sin, original or personal, and always more intimately united with her Son, offered Him on Golgotha to the Eternal Father for all the children of Adam, sin-stained by his unhappy fall." Pius viewed her compassion there as the basis for her role in redemption.
She is Most Holy Mother of all the members of Christ, and reigns in heaven with her Son, her body and soul refulgent with heavenly glory.
Mystici corporis did not receive much attention during the war years but became influential after World War II. It had rejected two extreme views of the Church.
Pius' statement of "profound grief" at the murder of the deformed, the insane, and those suffering from hereditary disease... as though they were a useless burden to Society" is a condemnation of the ongoing Nazi euthanasia program, under which disabled Germans were being removed from care facilities and murdered by the state as "life unworthy of life". It built upon the high-profile condemnations offered by the bishop of Munster, Clemens August Graf von Galen and others. It was followed, on 26 September 1943, by an open condemnation by the German Bishops which, from every German pulpit, denounced the killing of "innocent and defenceless mentally handicapped, incurably infirm and fatally wounded, innocent hostages, and disarmed prisoners of war and criminal offenders, people of a foreign race or descent".
The new role of the laity resulted in the foundation of numerous secular institutes with faithful members of all walks of life. Against significant difficulties and opposition from established religious orders, Pius XII issued in 1947 the Apostolic Constitution Provida Mater Ecclesia, which, for the first time in Church history, allowed lay people to form their own secular communities, and establish them within a newly established Canon Law framework. The Pope himself used the encyclical to encourage active participation of the laity by addressing a wide variety of groups and professional associations throughout the world.
For many years, it was thought that Vatican II had made one significant exception to Mystici corporis. The encyclical of Pope Pius stated that the Body of Christ is the Catholic Church. Pope Paul VI quoted Mystici corporis from Pius XII verbatim in his first encyclical Ecclesiam suam: "Consider, then, this splendid utterance of Our predecessor: 'The doctrine of the Mystical Body of Christ, which is the Church, a doctrine revealed originally from the lips of the Redeemer Himself...'" Pope Paul VI continues: "We wish to take up this invitation and to repeat it in this encyclical, for We consider it timely and urgent and relevant to the needs of the Church in our day."
The Council defined that the Church subsists in (subsistit in) the Body of Christ. This seemed to some theologians to relativize the identity of the one Catholic Church with the Body of Christ. Pope Paul VI, Pope Pius XII and all popes before him have taught complete identity. After some confusion over what subsistit in meant, the Vatican in 2007 clarified its position as being identical with Pope Pius XII. Leading Council theologians like Joseph Ratzinger and Henri de Lubac expanded on this.
Protestant theology since Martin Luther always rejected the Catholic view of the Church as one Church with both visible and invisible aspects, and with a Pope as the Successor of St. Peter. It employs instead the interpretation of the Bible as the sole source of orthodox theology (Sola Scriptura), yet its biblical interpretation of the mystical Body is different from the Catholic Church's doctrine as stated in Mystici corporis Christi: The Church receives all the graces from Christ its singular head prior to active participation. The doctrines of the mystical unity of the Catholic Church through engagement with the sacraments is thus rejected by most Protestants. However, Mystici corporis uses a biblical base for its teaching, and thus contributed to ecumenical dialogue with Protestantism, while still reaffirming that the Catholic Church is the one true Church. As well, in recent years, some Protestant theologians have returned to the doctrine of the "mystical body of Christ" afresh, often following the thought of Henri de Lubac in sympathetic fashion (Milbank, Suspended Middle, 2005; Boersma, Sacramental Ontology, 2009), and others have embraced the doctrine in a way which examines its development over time, and the call to ecclesiastical unity that it issues (Pecknold, Christianity and Politics, 2010).
The Eastern Orthodox churches share a tradition-based sacramental theology with the Catholic Church. Mystici corporis, establishing equality of all apostles under the Successor of Peter, instead of a supposed “papalist” Societas Perfecta, was viewed quite positively; even so, not all aspects were shared by all. Pope Pius recognized, and often criticized, an over-centralized papacy and related Church laws and regulations, as an obstacle to relations with the Eastern Orthodox churches. After issuing Mystici corporis, the Pope ordered a reform of the CIC Orientalis, the Canon Law for the Eastern Catholic Churches united with Rome. In its new constitutions, Eastern Patriarchs were made much more autonomous with regard to Eastern marriage law, civil law, laws governing religious associations, property law and other laws. These reforms were intended to provide for more independence to the Eastern Catholic Churches, establishing them as equal within the mystical body of Christ and supplying a model for Eastern Orthodox churches if they decide to reunite with the Catholic Church.