In 1958, as response to a query about Opus Dei, St. Josemaria Escriva simply said: “Opus Dei is a little bit of the Church.” The theologian Pedro Rodriguez remarked: “With that non-technical expression the founder doubtless meant to ignore the legal framework into which the Work was slotted at the time, in order to highlight better its essence as a ‘little bit of the Church.’ It struck me at the time (and still does) that he was pointing the way to understanding the ecclesiology of Opus Dei – getting to the very core of the question. To think and speak of Opus Dei soon sends us back to what the Church essentially is, to its saving riches. All that Opus Dei is, it is within the mystery of the Church. Consequently, to study Opus Dei on needs to have a good grasp of ecclesiology. The better we understand the Church, the better will we see how the ‘little bit’ fits in.”
To fast forward to the ontological core of the Church, then-Cardinal Ratzinger remarked: “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. A Marian understanding of the church is totally opposed to the concept of the Church as a bureaucracy or a simple organization. We cannot make the Church, we must be the Church. We are the Church, the Church is in us only to the extent that our faith more than action forges our being. Only by being Marian, can we become the Church. At its very beginning the Church was not made, but given birth. She existed in the soul of Mary from the moment she uttered her fiat. This is the most profound will of the Council: the Church should be awakened in our souls. Mary shows us the way.”
Cardinal Ratzinger, in another place, pointed to the personal-sacramental character of the Church: “There are some very real grounds to fear that the Church may assume too many institutions of human law, which then become the armor of Saul making it difficult for the young David to walk. We must always ascertain if institutions which were once useful still serve a purpose. The only institutional element the Church needs is the one given to it by the Lord: the sacramental structure of the people of God, centered on the Eucharist.”
The Church is a sacramental reality – again personal – of incorporation into Jesus Christ, Priest. The sacraments of Baptism and Orders. That means that the basic “structure” of the Church is the laity and the ministerial priesthood; or, since both sacraments are a participation in the one priesthood of Christ, the common priesthood of the faithful and the ministerial priesthood.
That being so, it is illuminating to see that Opus Dei is this and only this, i.e. “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, besides, the ‘ministerial’ consequences of the sacrament of Order.” So, Opus Dei is nothing but this relationship between – that is a communio (the “communio” that is the Church) – laity and ministerial priests, who relate as Bridegroom to Bride or Christ as Head to His Body, the Church. In a word, the essence of Opus Dei is the same essence as the Church. It is not a case of holding an institutional position in the Church as “a little bit,” but as “a little bit” of the whole Church. Opus Dei is the communio of laity and priests; the Church is the communio of laity and priests.
The identity becomes starker when one considers what Benedict XVI has clarified concerning the Kingdom of God as a Person. The Church is not coextensive with the Kingdom of God but the space and the sacrament of the Kingdom of God – which is the Person of Christ. The Kingdom of God is incubated from within the Church. The Kingdom of God is a Person with the face and name of Jesus of Nazareth (Redemptoris. Missio, #18). The Kingdom of God as Person is not a structure in the world. But as the divine Person of the Logos Who has assumed the human nature of Jesus of Nazareth, He is in the world. He, as Kingdom, is among us.
Ratzinger will say that: “the Kingdom of God cannot be observed, yet, unobserved, it is among those to whom he is speaking. It stands among them – in his own person. ‘Jesus in person is the “mystery of the Kingdom, made over as gift to the disciples by God.”’ In him the future is present, God’s Kingdom at hand, but in such a way that a mere observer, concerned with recording symptoms or plotting the movements of the stars, might well overlook the fact… Let us remember that “God’s Kingdom is an event not a sphere. Jesus’ actions, words, sufferings break the power of that alienation which lies so heavily on human life. In liberating people, they establish God’s Kingdom. Jesus is that Kingdom since through him the Spirit of God acts in the world.”
Now, notice the Content of the Founding of Opus Dei:
Alvaro del Portillo
1- the common priesthood of the faithful. LG #10.
2 - the universal call to sanctity. LG #39.
3 – unity of life
4 – professional work as occasion and means of personal sanctity and apostolate. LG #31.
5 – secularity as specific characteristic of the lay apostolate and the sharing of the lay faithful in the mission of the Church
6 –recognition of the personal freedom of the Christian in temporal questions
7 – Holy Mass as “center and root” of interior life
8 – configuration of the special dioceses or personal Prelatures for specific pastoral and apostolic activities.
(1985) “I believe… that the true time of Vatican II has not yet come, that its authentic reception has not yet begun: its documents were quickly buried under a pile of superficial or frankly inexact publications. The reading of the letter of the documents will enable us to discover their true spirit. If thus rediscovered in their truth, those great texts will make it possible for us to understand just what happened and to react with a new vigor. I repeat: the Catholic who clearly and, consequently, painfully perceives the damage that has been wrought in his Church by the misinterpretations of Vatican II must find the possibility of revival in Vatican II itself. The Council is his, it does not belong to those who want to continue along a road whose results have been catastrophic. It does not belong to those, who, not by chance, don’t know just what to make of Vatican II, which they look upon as a ‘fossil of the clerical era.’” (The Ratzinger Report, Ignatius  40)
Vataican II is intelligible only to one who already lives it. Therefore, practice the spirit of Opus Dei and read the Council Documents – especially Lumen Gentium, Gaudium et Spes and Dei Verbum. Communicate this to as many people as possible: The Kingdom of God is immanent and transcendent as the Person of Christ is immanent by His Humanity, and transcendent by His Person. Be Another Christ! You are that Kingdom insofar as you are "another Christ." This (you) is the coming of the Kingdom of God in time and space. Implement it by living the Holy Sacrifice of the Mass by work in the street.
 J. Ratzinger, “The Ecclesiology of Vatican II,” L’Osservatore Romano N. 4 -23 January 2002, 7.
 J. Ratzinger, 30 Days, No. 5 – 1998, p. 22.
 J. Ratzinger, “Eschatology,” CUA (1988) 34-35.