"Communio" is the central ecclesiological idea of Vatican II. The very opening paragraph of the Dogmatic Constitution, Lumen Gentium, describes the Church as "a sign and instrument of communion with God and of unity among all men" (LG 1). The Church is a living communion, drawing people into the life of Christ. She lives by that life and communicates it. "In the Church this communion of men with God... is the purpose which governs everything in her that is a sacramental means" (CCC 773).
In becoming Man, God has sought this communion with us. "All men are called to this union with Christ, who is the light of the world, from whom we go forth, through whom we live, and towards whom our whole life is directed" (LG 3). It is up to each one of us to respond freely to this call: to accept or neglect or reject communion with Christ. A positive response is an expression of charity: love for God which is necessarily love for all others, and so becomes an ecclesial charity. But charity alone is not enough to enter on and live within "communio"; we also need faith, precisely an ecclesial faith: "the invisible God... addresses men as his friends, and moves among them in order to invite and receive them into his own company. The adequate response to this invitation is faith" (CCC 142).
Just as one loves within the community of the Church, so one believes within the same community. The important thing is not to think 'on one's own', but to think with Christ, and with others in Christ. Christ wants his followers to be at one with each other, not only in charity - in love for him and for one another - but also in faith and government. It is easier to be one in charity than to be one in faith and discipline, for the latter not infrequently demands submission of the mind, "accepting" what one does not always see to be true or right or prudent.
Any merely human community has a limit in time; it inevitably comes to an end in death. The christian "communio" transcends time and death. It means oneness of heart and mind with all those who have shared the spirit of Christ throughout the ages. Hence derives the Catholic sense of Tradition as something living. In a certain sense, following Chesterton's idea, we can say that Catholicism is the truest "democracy" because it excludes no one; it gives a vote also to those who have gone before us, who belong not to the "past" but to a "present" much more permanent than ours, who are not dead but much more living than we are.
"Communio" cannot be lived in an exclusively spiritual or disincarnated way. Communion is never only with others, nor just directly with God. Within the plan of Christ, the full life-giving communion is achieved in and through the Church. It is therefore essential for each individual to maintain communion with the universal Church. He should do so by praying for others (cf. Communion of the Saints), by knowing church teaching as presented by the Magisterium, by following authoritative directives that come from the Holy See, etc. He should particularly be able to do so through his bishop, whose "authority must be exercised in communion with the whole Church under the guidance of the Pope" (CCC 895). It follows that each bishop has a special responsibility to defend and foster "communio": to ensure that his particular church is one with the Holy See, and his people one in Catholic faith and essential discipline. If a bishop were to allow a separatist spirit, he could lead astray those entrusted to him.
One should understand how "communio" harmonises with the christian philosophy of "personalism", also strongly present in Vatican II. The Council is in fact both "community-centered" amd "person-centered". There is no opposition. Personalism, as the Church presents it, is not inward-looking. Its thrust goes out to others, emphasizing that it is "only in a sincere giving of self that one can fully discover one's true self" (GS 24). Personalism understands that self-fulfillment lies in responding to values to be found outside one's self: in the world, in others, in God. In giving oneself, one centers more on others, one becomes more aware of their dignity and rights and more respectful towards them. Far from being opposed to the building of community, here is a spirit which powerfully helps it. It is secular individualism which is opposed to "communio", putting the individual self at the center of affairs, and wishing to subordinate all else to "self-expression" or "self-fulfilment", with little or no regard for the rights and freedoms of others. It is important to be able to recognize the difference between true personalism and individualism, for not a few programs or slogans, presented in the name of "renewal" or of Vatican II, have been individualist (and therefore hostile to "communio"), and not personalist. Individualistic approaches are incapable of renewing persons