The Church is the Mystical Body of her Head in virtue of being one body with him in a mystical marriage, in which he is Bridegroom and she is Bride. None of that is esoteric Catholic stuff; it's right there in the Bible. To be sure, in a culture wherein marriage has become one of the few contracts that may be and often is broken unilaterally without penalty, we have almost entirely lost the sense of what that mystical marriage is. Yet when fully manifest to all, as it is even now to the Church Triumphant, it will be and seem better than the most erotically charged sex. And there's a divinely instituted symbology here which already unites the Church Triumphant with the Church Militant. As Hans urs von Balthasar argued in his ground-breaking Office of Peter and the Structure of the Church, Mary and the papacy symbolize, and help to bring about by symbolizing, the reality of the Church as Bride united to her divine Head. Mary belongs to the Church Triumphant; the papacy, to the Church Militant. Note that the same kind of relationship is sacramentally embodied by the ordinary sacramental marriage of baptized couples, which St. Paul says is a mysterion of the relationship of Christ with his Church. Accordingly, and in the hope that others will afford me additional insights, I suggest that the most important area of theology today is ecclesiology, just as the most important area of theology in the Christianity's first millennium was Christology.
Just as the Church before the great schisms struggled for centuries to achieve deeper, more reliable formulations of orthodoxy, or right belief, about the individual Person of Jesus Christ, so today we have barely begun the struggle to develop such formulations about what St. Augustine called the whole Christ, i.e. that very Person in union with his Mystical Body. (Vatican II's Dogmatic Constitution on the Church, Lumen Gentium, took a few steps, but we don't seem to have kept moving in that direction.) That's part of what made the schisms possible, and its absence helps keep the schisms going. Everybody sort of remembers that things should not be so; not long after his ascension into heaven, Christ even asked the Church's deadliest opponent: "Saul, Saul, why do you persecute me?" There's an intimacy there that has become almost completely obscured by the sadder aspects of Church history and the bureaucratic aspects of the Church as organization. The union not fully realized, the "not yet," has crowded out the "already," the society that is "perfect" insofar as she is divinely instituted. Thus the true Church is more often seen as an eschatological ideal, or as present but invisible, or as visible but merely human—anything but the spotless bride that is one body with her groom. That's why Catholics still mostly use the pronoun 'it', not 'she', for the Church. By and large, all the baptized have allowed ourselves as sinful pilgrims "on the way" to forget what we already are: cells of the Mystical Body, married to her divine Head even when we are unfaithful, and thus collectively in the process of becoming more of what we already are.
Although we can forgo salvation by our individual choices, we cannot earn it. Unmerited salvation is offered to us gratuitously only in and through that Body whose agencies of grace enable our choices to work out our salvation. That is why it is dogma that "outside the Church there is no salvation." But how are we to conceive, for that purpose, that mystical union of which we are part? We are becoming gods, and will be if we persevere, but we are not after all God.
Pope John Paul the Great supplies a key to the answer by his having developed "the theology of the body" as a way to deepen the Church's understanding of the spiritual importance of sex and marriage. Its gravamen is that marriage is an embodied spiritual "intercommunion" established by free, complete, mutual self-gift. It is not ethereal, not abstract, not just an ideal. It is attainable in the flesh. When authentic, it is embodied in a couple's common life generally, and in particular by their sexual relationship. The visible difference between the male and female bodies is nuptially significant, not merely as an evolutionary device for reproduction but as the sign and instrument of spiritual intercommunion in marriage. That's what makes it possible for sex to be making love and for reproduction to be procreation. When faithfully lived out and embodied, such a union brings about what it symbolizes: ordinarily and physically, in the procreation and education of children, and spiritually in that love which cements and vivifies the family. Thus, when exemplified as God wills, sex, marriage, and family are sacramental.
The same goes for the Church, which, in Lumen Gentium's words, is the "sacrament of unity and salvation" for all humanity. She is constituted by the intercommunion of the risen Christ , the Bridegroom, with his people, who collectively constitute his Bride. Certain women can experience that individually, as have certain nuns and mystics. But women and men together, as the "people of God," are all called to "receive" Christ into themselves—at the Eucharist, quite literally—so that his life, eternal life, can be brought forth within them to transform them and the world at large in agape: total, disinterested divine love.
Of course this complex of relationships does not have the same physical limitations as biological ones. Collectively we are Bride, and in union with our Bridegroom we bring forth children. But the "children" of the divine Bridegroom are all those who are transformed by that union: we ourselves, individually. And the better children of God we are, the more effectively we will make Christ palpable in and for the world. Thus we are all, men as well as women, called to imitate the God-bearer, the Theotokos: we are to give birth to Christ anew within ourselves and thus within the world. That involves death to sin and rising to life in the Spirit. The entire process recapitulates the Incarnation, Passion, and Resurrection of Christ, the divine Hero who rescues us from all that would keep us from our destiny as celebrants and givers of life.
But the process needs a visible regulator. Von Balthasar put it well when he expounded in his book on the "Marian" and "Petrine" charisms in the Church. Those form a spiritual polarity that symbolically recapitulates the intimate relationship between Christ and the Church and thus serves, sacramentally, to cement her unity with him. The Marian charism of receptivity to God, submissive fidelity to Jesus Christ, and fruitfulness in bearing him into the world is fully shared by every member of the faithful, from the bottom to the top; it just is the superordinate, multi-layered gift of grace empowering the Church to be the Bride of Christ and thus, by the power of the Holy Spirit, bear his children into the world. But the Petrine charism of teaching and governing authority, invested primarily in bishops and derivatively in priests and deacons, exists to facilitate and serve the Marian by efficaciously signifying Christ as the self-immolating Bridegroom and Head of the Church. Hence the hierarchical nature of the Church—her hieros arché or “sacred order”—is itself a hieros gamos, a sacred marriage. Extending that truth to the horizon is the teaching of Lumen Gentium: "By her relationship with Christ, the Church is a kind of sacrament of intimate union with God and of the unity of all mankind, that is, she is a sign and an instrument of such union and unity." As the sacrament of such union, the Church is also the sacrament of salvation (I prefer the Eastern term theosis) for humanity.
Sacramentality always entails what one of my teachers, the late Theodor Gaster, called "punctualization." Mary is the punctualization of the charism of the believer and disciple, and the Marian dogmas each illustrate aspects of that. Thus the Immaculate Conception is, to a supreme and paradigmatic degree, what each believer is at baptism; the virginal conception of Jesus signifies that the new, transforming life we receive at baptism comes only from above; the Assumption of Mary into heaven proleptically anticipates what will happen to each of the saved on the Last Day. Even the widely misunderstood teaching that Mary is "Mediatrix" of all graces is but a logical corollary of her role as Mother of the Church, which is the divinely ordained ark of the salvation that is by grace alone. The papal office is in its turn intended to be the "punctualization" of the Petrine charism for the episcopal college itself, and by that means for the Church as a whole. In both cases, the punctualization is sacramental inasmuch as it helps to bring about what it signifies.
I suppose what's really lacking is appreciation of the sacramental character of the Church precisely as people of God and Mystical Body. We tend to think of church as the place you go to "get" the sacraments, as if they were pumps at a spiritual filling station. But the Church herself is the sacrament affording the context for all the others. And she is that as God's wife. We need an ecclesiology to bring that alive for people.