23 This collegial union is apparent also in the mutual relations of the individual bishops with particular churches and with the universal Church. The Roman Pontiff, as the successor of Peter, is the perpetual and visible principle and foundation of unity of both the bishops and of the faithful. The individual bishops, however, are the visible principle and foundation of unity in their particular churches, fashioned after the model of the universal Church, in and from which churches comes into being the one and only Catholic Church. For this reason the individual bishops represent each his own church, but all of them together and with the Pope represent the entire Church in the bond of peace, love and unity. The individual bishops, who are placed in charge of particular churches, exercise their pastoral government over the portion of the People of God committed to their care, and not over other churches nor over the universal Church. But each of them, as a member of the episcopal college and legitimate successor of the apostles, is obliged by Christ's institution and command to be solicitous for the whole Church, and this solicitude, though it is not exercised by an act of jurisdiction, contributes greatly to the advantage of the universal Church. For it is the duty of all bishops to promote and to safeguard the unity of faith and the discipline common to the whole Church, to instruct the faithful to love for the whole mystical body of Christ, especially for its poor and sorrowing members and for those who are suffering persecution for justice's sake, and finally to promote every activity that is of interest to the whole Church, especially that the faith may take increase and the light of full truth appear to all men. And this also is important, that by governing well their own church as a portion of the universal Church, they themselves are effectively contributing to the welfare of the whole Mystical Body, which is also the body of the churches. The task of proclaiming the Gospel everywhere on earth pertains to the body of pastors, to all of whom in common Christ gave His command, thereby imposing upon them a common duty, as Pope Celestine in his time recommended to the Fathers of the Council of Ephesus. From this it follows that the individual bishops, insofar as their own discharge of their duty permits, are obliged to enter into a community of work among themselves and with the successor of Peter, upon whom was imposed in a special way the great duty of spreading the Christian name. With all their energy, therefore, they must supply to the missions both workers for the harvest and also spiritual and material aid, both directly and on their own account. as well as by arousing the ardent cooperation of the faithful. And finally, the bishops, in a universal fellowship of charity, should gladly extend their fraternal aid to other churches, especially to neighboring and more needy dioceses in accordance with the venerable example of antiquity. By divine Providence it has come about that various churches, established in various places by the apostles and their successors, have in the course of time coalesced into several groups, organically united, which, preserving the unity of faith and the unique divine constitution of the universal Church, enjoy their own discipline, their own liturgical usage, and their own theological and spiritual heritage. Some of these churches, notably the ancient patriarchal churches, as parent-stocks of the Faith, so to speak, have begotten others as daughter churches, with which they are connected down to our own time by a close bond of charity in their sacramental life and in their mutual respect for their rights and duties. This variety of local churches with one common aspiration is splendid evidence of the catholicity of the undivided Church. In like manner the Episcopal bodies of today are in a position to render a manifold and fruitful assistance, so that this collegiate feeling may be put into practical application.
Source: Lumen Gentium (Vatican.va)