Encyclical letter, "Mystery of the Faith," promulgated by Pope Paul VI on Sept. 3, 1965. In light of the initiative of the Second Vatican Council to reform the sacred liturgy of the Church, Mysterium fidei provides clarification and direction concerning the doctrine and worship of the Eucharist.
The introduction establishes that when treating the reform of the liturgy, Vatican II "considered nothing to be more important than urging the faithful to participate actively and with sound faith and with utmost devotion in the celebration of this most holy mystery; to offer it with the priest to God as a sacrifice for their own salvation and for that of the whole world, and to find in it spiritual nourishment" (no. 2). Following these points, the pope affirms, "the mystery of the Eucharist is at the heart and center of the liturgy itself" (no. 3). He explains that with regard to the Eucharist the council "wished to make evident the indissoluble relationship between faith and devotion" (no. 4).
In part 2, "Reasons for Pastoral Concern and Anxiety," Paul VI registers alarm at "opinions" about the Mystery of the Eucharist that circulate "in written or spoken word" (no. 10). As a corrective to these judgments, he decrees that the following interpretations are "not allowable": "to emphasize … the Mass 'of the community' to the extent of disparaging Masses celebrated in private; or to stress the sign value of the sacrament as if the symbolism … expresses fully and exhaustively the meaning of Christ's presence; or to discuss the mystery of transubstantiation without mentioning the changing of the … bread … and wine … as stated by the Council of Trent; or finally, to propose and to act on the opinion according to which Christ the Lord is no longer present in the consecrated hosts left after the celebration of the sacrifice of the Mass" (no. 11).
Part 3, "The Holy Eucharist Is a Mystery of Faith," highlights the concept of mystery : the Eucharist is a "very great mystery" and a "Mystery of Faith" (no. 15). Believers must approach the Eucharistic mystery with "humble respect, not following human [rational] arguments … but adhering firmly to divine revelation" (no.16). Investigations of this unparalleled mystery should be guided by "the magisterium of the Church" (no. 22). In addition to "safeguarding" the integrity of the Eucharistic mystery itself, its "proper mode of expression" (no.23) must also be safeguarded. Thus, the Church has established a "rule of language," which it has "confirmed… with the authority of the councils" (no. 24).
In part 4, "The Mystery of the Eucharist Is Verified in the Sacrifice of the Mass," the pope reviews traditional Catholic doctrine related to the Mass. Here, he indicates that "the whole Church … in union with Christ in His role as Priest and Victim, offers the Sacrifice of the Mass and is offered in it." He also underscores, "the distinction between the universal priesthood and the hierarchical priesthood is one of essence and not merely one of degree" (no. 31). Explaining the "public and social nature of every Mass," the pope says, "Mass is not something private; it is an act of Christ and of the Church." As such, "every Mass is offered not for the salvation of ourselves alone, but also for that of the whole world" (no. 32).
In part 5, "In the Sacrifice of the Mass Christ Is Made Sacramentally Present," the pope teaches that "sacrifice and Sacrament pertain inseparably to the same mystery." The foundational principle of this instruction is that "in an unbloody representation of the Sacrifice of the Cross and in application of its saving power, in the Sacrifice of the Mass the Lord is immolated when, through the words of consecration, He begins to be present in a sacramental form under the appearances of bread and wine" (no. 34). Of the various ways Christ is present in the Church, the Sacrament of the Eucharist "surpasses all the others." To refer to Christ's presence in the Eucharist as "real" does not "exclude all other types of presence as if they could not be 'real' too," but is "presence in the fullest sense," that is, "it is the substantial presence by which Christ, the God-man, is wholly and entirely present." It is "wrong to explain this presence by … recourse to the 'spiritual' nature … of the Glorified Body of Christ … or by reducing it to a kind of symbolism" (no. 39).
Part 6, "Christ Our Lord Is Present in the Sacrament of the Eucharist by Transubstantiation," reiterates Catholic teaching on transubstantiation: "the voice of the teaching and praying Church…. assures us that the way Christ is made present in this Sacrament is none other than by the change of the whole substance of the bread into His Body, and of the whole substance of the wine into His Blood, and that this … change the Catholic Church rightly calls transubstantiation." As a consequence of this substantial change, "the species of bread and wine … take on new meaning and a new finality, for they no longer remain ordinary bread and ordinary wine, but become the sign of something sacred, the sign of a spiritual food." Thus, transformed bread and wine "contain a new 'reality' which we may justly term ontological." Transubstantiation alters the "objective reality" of the bread and wine, "since after the change of the substance or nature of the bread and wine into the Body and Blood of Christ, nothing remains of the bread and wine but the appearances, under which Christ, whole and entire, in His physical 'reality' is bodily present" (no.46).
Part 7, "Latreutic Worship of the Sacrament of the Eucharist," recalls that the Catholic Church "has always offered and still offers the cult of Latria to the Sacrament of the Eucharist, not only during Mass, but also outside of it, reserving Consecrated Hosts … exposing them to solemn veneration, and carrying them processionally" (no. 56). The pope highlights the feast of Corpus Christi as a testimony to this veneration, which continues to give rise to inspirational Eucharistic pieties. Through them, the Catholic Church strives "to do homage to Christ … to thank Him … and to implore his mercy" (no. 63).
The concluding section, "Exhortation to Promote the Cult of the Eucharist," exhorts persons entrusted with the care of believers "to preserve this faith in its purity and integrity" and to "promote the cult of the Eucharist" (no. 64).