To understand this doctrine of the Church is to have apprehended, at the same time, the basis and reason of the liturgy. The Church, as the mystic body of Christ, lives her life in the liturgy; and it is a life of magnificent prayer. Following the example of the divine Christ, the mystic Christ has always prayed, now in the garden, now on the mountain, now in tears like Magdalen, now in joy like David before the ark of the covenant. And in all these variant actions of prayer, the Church is intensifying her sanctity. Those members who use the liturgy as a means of divine glorification and personal sanctification, are making themselves more perfect members of the mystical Christ, and are co-operating in the building up of the body of Christ, ’till we all attain to the unity of faith and of the full knowledge of the Son of God, to the perfect man, to the full measure of the stature of Christ’ (“The Mystic Body of Christ”, OF I.7, 202).
Polycarp’s disciple, St. Irenaeus, developed the teaching into a beautifully systematic theology of the Church. In his Adversus Haereses, he challenges the Gnostic heretics with a teaching he himself insists is Apostolic in origin, a teaching which practically unites all branches of theology into the Mystical Body. St. Irenaeus calls it the “recapitulation.” All things are recapitulated in Christ. All human history, from Adam to his last son are “recapitulated” in Christ so that what went wrong in Adam will be made right in the Second Adam.
Some of the most difficult passages in St. Paul are rendered less so as long as the reader keeps in mind his obsession with this doctrine of the Mystical Body. It is speculated that St. Paul would have written almost exclusively of this mystery in his writings, had not pastoral concerns prevented him from doing so. But even then, when problems arise in the various local churches, their Apostle takes this occasion to exhort them to unity “in Christ,” thus applying very elevated theological concepts to concrete situations and laying the only sound foundation for Charity toward one’s neighbor. I will close this little section on St. Paul by citing one of these pastoral exhortations toward unity in which the Apostle uses the Mystical Body doctrine to teach a lesson in Christian Charity. In his first epistle to them, St. Paul tells the Corinthians the following:
It follows, therefore, that the Mystical Body of Christ isthe Roman Catholic Church, whose members are all those who have been baptizedand who at least externally practice and profess the true faith. Commentatorson the make specialnote of the fact that, after centuries of controversy on the subject, the Popehas authoritatively approved Bellarmine's doctrine on the minimum essentialsfor membership in the Mystical Body—which reads like a paraphrase fromthe third book of St. Robert's . In the words of Pope Pius XII, "only those arereally to be included as members of the Church who have been baptized andprofess the true faith and have not unhappily withdrawn from Body-unity, or forgrave faults been excluded by legitimate authority. For in one Spirit were weall baptized into one Body." 
THE CHURCH : Mystical Body of Jesus Christ
In general, however, when Bellarmine speaks of the MysticalBody, he has in mind only the first of its three branches, the ChurchMilitant—or, in other words, the visible organization of the RomanCatholic Church. Thus, in treating the delicate question of occult infidels, herefutes the doctrine of Calvin who held that, if a baptized person has lost thevirtue of faith, in spite of his external profession of belief and conformitywith Christian practice he is no longer a member of the organic Body of Christ."It is certainly true," he admits, "that a sincere faith and not its mereexternal profession is required if we are to be internally united to the Bodyof Christ, which is the Church . . . . But even the man who makes only anoutward profession along with the rest of the faithful is a true member, albeita dry and dead member, of the Body of the Church." 
Ch 2: The Mystical Body of Christ, the Church | Christian ...
41. They, therefore, walk in the path of dangerous error who believe that they can accept Christ as the Head of the Church, while not adhering loyally to His Vicar on earth. They have taken away the visible head, broken the visible bonds of unity and left the Mystical Body of the Redeemer so obscured and so maimed, that those who are seeking the haven of eternal salvation can neither see it nor find it.
the Mystical Body of Christ, or the four walls of our parish church.
war as a tearing of the Mystical Body involves two interrelated points. First, the unity present in the Mystical Body exists in opposition to nationalism and war in all its forms; if the humanity of Christ is singular, then social identity located elsewhere exists as an idolatrous simulacrum of the Mystical Body. Consequently, and secondly, the various practices and teachings of the Church should emphasize this transnational, transpolitical unity, in contrast to the divisive sociality perpetuated by war and nationalism. In her famous “In Peace is My Bitterness most Bitter”, Day argues that national interests are opposed to the practices of the Church in precisely this manner: