Opus Dei is analogical to a diocese (the difference being that Opus Dei is not geographical but "personal"). As a diocese instantiates the Universal Church in a geographical territory with a cathedral church, the See of the bishop, so also Opus Dei has a "personal" church - Santa Maria de la Paz - encompassing the tomb of the person of the founding Prelate, St. Josemaria Escriva, endued in his successors with episcopal powers.
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The significance of the solemn dedication of the Prelatic Church, Our Lady of Peace, at the headquarters of Opus Dei in Rome rests on the fact that Opus Dei as a personal prelature is analogous to a diocese (particular Church) and therefore has a prelatic Church with a prelatic See. It is not a diocese and therefore does not locate the universal Church in a geographical location, but because it is analogous to a diocese it does have its church and see. The Prelature being "personal" and not geographical, the oratory Santa Maria de la Paz was chosen because it contains the personal remains of the Founder, St. Josemaria Escriva.
When all is said and done, “Opus Dei, as a Christian community, is not a particular Church, but rather a transdiocesan, universal convocation of the faithful that transcends particular Churches because the authority of the Prelate is not sacramental conferred by a juridical act of the Pope. If the authority were sacramental, it would be limited to a specific geographical region. As it hangs off the sacramental reality of the papacy, it is universal and transdiocesan in scope. However, it is episcopal in function in that it convokes, engenders [as a "Father"] and directs the ontological reality of the communio that is Opus Dei comprised of both laity and ministerial priests that are unum in vocation, free and autonomous in secular, professional life, and directed to a common apostolic mission in all the dioceses. Because of this, it has been fitting that the Prelate be consecrated a bishop and pertain to the college of bishops. Concluding then, as dioceses have cathedral churches for the bishop, by analogy to dioceses (particular churches), Opus Dei has its own Prelatic Church.
Opus Dei’s internal structure replicates “the aboriginal relationship obtaining in the Church between christifideles- called to live out the requirements and implications of their baptism – and sacred ministers, who bring in… the ‘ministerial’ consequences of the sacrament of Order.” In the words of St. Josemaria Escriva, “Opus Dei is a little bit of the Church.”
Opus Dei: “Analogous” to a Particular Church (Diocese)
This “aboriginal relationship” of baptized laity and ordained ministerial priests has historically taken the form of “particular churches,” principally dioceses. Rodriguez says “the particular Church, precisely since it is an element of the ‘Church’s essential structure’… historically takes a variety of forms de iure ecclesiastico) those described by canon 368: diocese, territorial prelatures, and so forth). These forms pertain to the Church’s definite et mutabilis form, and the communion of all of them in the universal Church makes up the Corpus Ecclesiarum of Vatican II.”
Rodriguez goes on: “The ‘eminent’ character of the particular Church makes of it a natural analogue for understanding theologically those other institutions that, without being particular Churches, do respond to the structural element ‘christifideles/sacred ministry.’ These other forms of Christian community are structurally canonical creations (de iure ecclesiastico) that thus represent an historical development of the oft-cited structural dimension of the Church (‘christifideles/sacred ministry’). As we have seen, this dimension is certainly primary, basic, aboriginal (de iure divino)” (emphasis mine).
In the case of Opus Dei, the Church “has discerned the foundational charism and the social arrangement immanent in the Christian community born of that charism” (emphasis mine).
Opus Dei: Not a Particular Church, But Personal Prelature
Rodriguez says: “Opus Dei is not a particular Church, but is has in common with the particular Church a certain level of ‘theological substance.’” That “theological substance” is the sacramental, and therefore ontological, reality of laity and priests that form a communio of the two as constitutively interdependent and forming the secular reality of the Body of Christ. I say “secular” because of the “theonomous” freedom that derives from the human will of Christ as the true will of the divine Logos. The human will of Jesus of Nazareth never ceased to be human and free when assumed by the divine Person of the Logos. On the contrary, it became fully itself ("compenetrated") as human free will in the Son’s obedience with it to the Father in His death on the Cross. That freedom of autonomy or “theonomy” is our freedom as baptized and ordained into His priesthood of Self-gift -- freedom, of course, meaning the freedom of the divine Person as Self-gift to the Father. That freedom is revealed in its fullness in Christ crucified (Self-gift) to which we as laymen and priests are called.
It is critical to understand that the Prelate of Opus Dei, although he has episcopal powers by juridical act of the Pope, does not situate the universal Church in a geographical place. “Rather, his role is to gather faithful and priest in Opus Dei in order to carry out its own particular apostolic mission, which is one of a universal scope...” However, because the primary mission of the Prelate is to generate sons and daughters as laity and priests into the self-gift of the communio that is the Work, his most important task as Prelate is to be “Father.” Hence, “what is decisive is neither his ‘jurisdiction’ nor their obedience. Rather, what truly defines Opus Dei’s prelate is his ‘fatherhood,’ his role as a pastor who is a father to all the Prelature’s faithful… The prelate’s role in the life of Opus Dei deeply configures the prelature.”
The core understanding of the particular Church demands that we back up to the reality of the Church Herself. What is she? Benedict at one point said that the Church is not a “what” but a “who:” “The Church is not an apparatus, nor a social institution, nor one social institution among many others. It is a person. It is a woman. It is a Mother. It is alive.” The Church is a “who” as the “space” in which the “I” of Christ is engendered.
Hence, there is no salvation outside of the Church because there is no salvation outside of Christ. The Church, the true Church, is Jesus Christ Himself who is both God and man. There is only salvation through Him Who is God/man. He as God/man reveals Himself to be the Bridegroom and the Church is His Bride. The one flesh union of the Bridegroom and the Bride is the Eucharist whereby the Bridegroom becomes the Head and the Bride becomes the Body. The Oneness made up of Head and Body form the one single Whole Christ. The Body that is the Church of Jesus Christ, comprised sacramentally by Baptism and Orders into lay faithful and ministerial priests, “subsists” in the Catholic Church. This needs explanation. Then-Cardinal Ratzinger expatiated:
“Council: 'Subsistit In' Explains Church as Concrete Subject"
"At this point it becomes necessary to investigate the word subsistit somewhat more carefully. With this expression, the Council differs from the formula of Pius XII, who said in his Encyclical Mystici Corporis Christi: 'The Catholic Church "is" (est) the one mystical body of Christ'. The difference between subsistit and est conceals within itself the whole ecumenical problem. The word subsistit derives from the ancient philosophy as later developed in Scholastic philosophy. The Greek word hypostasis that has a central role in Christology to describe the union of the divine and the human nature in the Person of Christ comes from that vision. Subsistere is a special case of esse. It is being in the form of a subject who has an autonomous existence. Here it is a question precisely of this. The Council wants to tell us that the Church of Jesus Christ as a concrete subject in this world can be found in the Catholic Church. This can take place only once, and the idea that the subsistit could be multiplied fails to grasp precisely the notion that is being intended. With the word subsistit, the Council wished to explain the unicity of the Catholic Church and the fact of her inability to be multiplied: the Church exists as a subject in historical reality.”
The development of this “Who” – the “I” of Christ - is the achievement of salvation, or eternal life. This takes place only by the giving of self dynamized by the love of Christ for each one that is called “grace,” that occurs in the communio of the Church. This communio - the "I" of the Church - has as its aboriginal form the relation of lay faithful to priest and priest to layman, or the need of the Church of Mary for the Church of Peter, and the Church of Peter for the Church of Mary. This aboriginal relationship in Opus Dei where both layman and priest have the same vocation (equally sharing in the one priesthood of Christ) to sanctity but are irreducibly different (as male and female) is the “Gestalt” of the Church that is “prior” to the canonically established “religious” life that was a later development.
Chronological Priority of Secularity to the “Religious” Canonical State
Rodriguez: “The ‘personal prelature’ formula, appropriating as it does an institution that expresses the internal dimension of the Church’s basic structure, locates Opus Dei within the order of ecclesial realities prior (emphasis mine) to the historically verified tensions between canonically secular and religious persons or institutions within the Church, with their legal and social consequences. A personal prelature simply indicates that in Opus Dei there is no consecration other than that of character-imprinting sacraments (emphasis mine), which alone, as we have said repeatedly, structure this kind of institution. Opus Dei, as an institution, enjoys the secularity proper to the Church’s hierarchical [meaning: “sacred origin”] institutions. Consequently, all the members of Opus Dei are secular; the lay faithful are ordinary faithful who live their Christian life from the lay or secular status characteristic of all lay people; and the priests incardinated in the Work are simply secular priests, who find their place in the prelature – and therefore in the Church- through the ‘ministeriality’ that structurally defines sacred ministers.”
 Pedro Rodriguez, “The Place of Opus Dei in the Church,” Opus Dei in the Church, Scepter (1994) 38.
 Ibid 1.
 Ibid 45.
 Ibid 47.
 Cf. Veritatis Splendor #85.
 Pedro Rodriguez… 55.
 Ibid 56.
 J. Ratzinger, “The Ecclesiology of Vatican II,” L’Osservatore Romano N. 4 – 23 January 2002, 7.
 J. Ratzinger, “Ecclesiology of the Constitution on the Church, Vatican II, ‘Lumen Gentium,’” L’Osservatore Romano, 19, September 2001 pp. 5-8.
 See John Paul II, “Mulieris Dignitatem” ftn. #55.
 Pedro Rodriguez, op. cit, 42.