The Fact: On February 24, 1947 “The Priestly Society of the Holy Cross and Opus Dei” was granted pontifical approval de iure as a Secular Institute – in fact, the first – which was formalized in the Decretum laudis, “Primum Institutum.” De facto, Opus Dei’s real nature transcended this juridical conceit, but had to wait for the Second Vatican Council and its provision for the "personal prelature" (see below).
The Secular Institute was not a happy fit for Opus Dei because a) Opus Dei remained dependent upon the Sacred Congregation for Religious (whereas Opus Dei is characteristically “secular”); and b) it set Opus Dei in the context of the states of perfection which heretofore involved the taking of vows, living common life as separation from the world, and wearing distinguishing dress to indicate pertinence to that state. All of this was foreign to the spirit received by St. Josemaria Escriva which he summed up like this: “Understand this well: there is something holy, something divine hidden in the most ordinary situations, and it is up to each one of you to discover it.” The immense advantage of such pontifical approval, however, was the ability to expand internationally to all dioceses pending the approval of the local Bishops.
The Obstacle: The Code of Canon Law of 1917 was the first compilation of the law of the Church. At the time Escriva was looking for a juridical solution for Opus Dei, the Code “was at its apex. After a period in Church history when a consensus had been reached that the old sources of legislation lacked the clarity and vitality to confront the grave and great questions facing the Church… the Codex was seen as the answer. It would foster the formation and improvement of the clergy to direct the ecclesiastical organization, while offering improved or new channels for pastoral action. This view was solidly grounded. But one must also recognize that, without doubting the Code’s undeniable advantages, it was sometimes applied too rigidly. The traditional flexibility of Canon Law to welcome renewing and rejuvenating movements in the pastoral life of the Church was curtailed. Some even claimed that what was not regulated or recognized in the Codex could have no citizenship in the life of the Church. A phrase circulating in Rome and attributed to… the Secretary of State until 1930 and principal mover of the new Code, had acquired the status of a maxim: quod non est in Codice non est in mundo: what is not found in the Code does not exist in the world.”
The Intention of Escriva: “What did I want? A place for the Work within the law of the Church, in accordance with the nature of our vocation and the demands imposed by the expansion of our apostolates; a full approval from the Magisterium for our supernatural way, including a clear and explicit description of our spiritual character. The growth of the Work, the multitude of vocations of people of every class and walk of life, all this which was a blessing from God urged me to try to obtain – from the Holy See – full juridical approval for the way which our Lord had opened up.”
The Reality That Did Not Fit: The radicality equality of vocation of laymen and priests who formed the same juridical class. “Thus to the question, What is the ecclesiological nature of Opus Dei? One could reply: `It is an institution whose internal structure replicates the basic ecclesial articulation between the common priesthood of the faithful, possessed by virtue of baptism, and the ministerial or hierarchical priesthood, possessed by the clerics incardinated in it.’
“So, what we find in Opus Dei, different yet complementing one another, are the two ecclesial forms of participating Christ’s priesthood. We find both the `substantial’ priority of Opus Dei’s lay faithful, at whose service is the priestly ministry, and the `functional’ priority of the sacred ministry, in whose head (the prelate) resides the sacra potestas that governs the prelature. The clergy’s `function’ priority was described by the founder when he said that the ministerial priesthood `impregnates with its spirit our personal life and all our apostolic work.’…Graphically, the founder told the Work’s priests that their task is to be a `carpet’ for others. He wrote: `In Opus Dei we’re all equal. There’s only a practical difference: priests are more bound to place their hearts on the floor like a carpet, so that their brothers and sisters may tread softly.’” 
Vatican II (Lumen Gentium): The Key to the Radical Equality of All the Baptized:
“The basis of this whole problem and the key to its solution lies in one incontrovertible fact, emphasized with unprecedented vigour by the Second Vatican Council, namely that all persons who belong to the Church have a common fundamental legal status, because they all share one and the same basic theological condition land belong to the same primary common category. All the faithful, from the Pope to the child who has just been baptized, share one and the same vocation, the same faith, the same faith, the same Spirit, the same grace. They are all in need of appropriate sacramental and spiritual aids; they must all live a full Christian life, following the same evangelical teachings; they must all lead a basic personal life of piety – that of children of God, brothers and disciples of Christ – which is obligatory for them before and above any specific distinction which may arise from their different functions within the Church. The all have an active and appropriate share – within the inevitable plurality of ministries – the single mission of Christ and of the Church. Therefore it follows logically that within the Church all members have certain fundamental rights and obligations in common.”
The Personal Prelature
The Final Step: The Prelature as guardian of 1) the oneness of the subjective vocation of both laity and priests forming a "communio": each, being sacramentally irreducible [by Baptism and Orders], makes the total gift of self being dynamized to do so by the pastoral charity (fatherhood) of the Prelate; and 2) secularity as “characteristic” whereby the world of work and family is the occasion of the self-giving.
The Conciliar Decree Presbyterorum Ordinis #10 reads: “Where the nature of the apostolate demands this, not only the proper distribution of priests should be made easier but also the carrying out of special pastoral projects for the benefit of different social groups in any region or among any race in any part of the world. For this purpose there can with advantages be set up some international seminaries, special dioceses, or personal prelatures and other institutions to which, by methods to be decided for the individual undertaking and always without prejudice of the rights of the local ordinaries, priests can be attached or incardinated for the common good of the whole Church.”
On August 6, 1966, Paul VI wrote the Apostolic Letter Ecclesiae Sanctae for the implementation of, in our case, Presbyterorum Ordinins #10: It read:
“(4) Furthermore, in order to accomplish special pastoral or missionary tasks for various regions or social groups requiring special assistance, prelatures may usefully be established by the Apostolic See. These would consist of the secular clergy specially trained and under the rule of a prelate of their own and governed by statutes of their own.
“It would be the duty of such a prelate to erect and govern a seminary for the suitable training of students. He would have the right to incardinate such students under the title of service to the prelature and to promote them to Orders.
“The prelate should show care for the spiritual life of those he promoted under the title mentioned above, and for the continuance of their special formation and their particular ministry, by making arrangements with the local ordinaries to whom they are sent. He should also make provision for suitable means of living either by such agreements as are mentioned above or out of the resources of the prelature or by appropriate subsidies. He should also make provision for those who through illness or other reasons are obliged to relinquish their post.
“There is no reason why laymen, whether celibate or married, should not dedicate their professional service, through contracts with the prelature, to its works and enterprises.
“Such prelatures shall not be erected without first hearing the views of the episcopal conferences of the territory in which they will serve. In the exercise of their function care is to be shown that the rights of the local ordinaries are not infringed and that close relations are kept with the episcopal conferences at all times.”
John Paul II erects Opus Dei as a personal prelature: November 28, 1982: Apostolic Constitution Ut Sit of universal extension.
 St. Josemaria Escriva, “Passionately Loving the World,” Scepter (2002) 5.
 “The Canonical Path of Opus Dei,” Scepter (1994) 139.
 Letter 25 January 1961.
 Pedro Rodriguez, “The Place of Opus Dei in the Church,” Opus Dei in the Church (Scepter (1994) 38.
 Alvaro del Portillo, “Faithful and Laity in the Church,” Ecclesia Press (Shannon, Ireland) (1972) 19.