On 29 June 1943, Pope Pius XII issued his encyclical Mystici Corporis Christi which presented the teaching of the Catholic Church as the Mystical Body of Christ.
In this encyclical, Pius XII asserts that complete participation in the Church and in the Body of Christ belongs to those who are in communion with the visible representantive of Christ, the Pope. These faithful are brought into union with Christ through the Eucharist, a union which is marked by the faithful working toward perfection and Christ working in the faithful. The Church is the Mystical Body of Christ because the union with Christ is not a purely physical or spiritual union, rather it is a supernatural union. The Mystical Body of Christ is truly visible on Earth as it is at the same time truly united with Christ in heaven.
The encyclical stresses the responsibilities of all the faithful – both lay and religious – to work toward the perfection of the Body of Christ. So, as a result of this encyclical, many new associations of lay faithful were formed.
The Mother of God also plays an important role in the nature of the Body of Christ as a mystical union of the faithful as she who is the Mother of the Son is also the Mother of all those who belong to the Son. Having been assumed into heaven, she now intercedes for all her children.
And if at times there appears in the Church something that indicates the weakness of our human nature, it should not be attributed to her juridical constitution, but rather to that regrettable inclination to evil found in each individual, which its Divine Founder permits even at times in the most exalted members of His Mystical Body, for the purpose of testing the virtue of the Shepherds no less than of the flocks, and that all may increase the merit of their Christian faith. For, as We said above, Christ did not wish to exclude sinners from His Church; hence if some of her members are suffering from spiritual maladies, that is no reason why we should lessen our love for the Church, but rather a reason why we should increase our devotion to her members. Certainly the loving Mother is spotless in the Sacraments by which she gives birth to and nourishes her children; in the faith which she has always preserved inviolate; in her sacred laws imposed on all; in the evangelical counsels which she recommends; in those heavenly gifts and extraordinary grace through which with inexhaustible fecundity, she generates hosts of martyrs, virgins and confessors. But it cannot be laid to her charge if some members fall, weak or wounded. In their name she prays to God daily: “Forgive us our trespasses;” and with the brave heart of a mother she applies herself at once to the work of nursing them back to spiritual health. When, therefore, we call the Body of Jesus Christ “mystical,” the very meaning of the word conveys a solemn warning. It is a warning that echoes in these words of St. Leo: “Recognize, O Christian, your dignity, and being made a sharer of the divine nature go not back to your former worthlessness along the way of unseemly conduct. Keep in mind of what Head and of what Body you are a member.”
The encyclical Mystici Corporis Christi