The mark of oneness reflects the unity of the Trinity. The Holy Spirit, the bond of love between the Father and the Son, unites all the members of the Church as the one People of God. The Church professes one Lord, one faith, and one Baptism and forms one body (cf. CCC, no. 866) under the leadership of the Holy Father, successor to Peter the Apostle. Within the Church there is a diversity of races, nations, cultures, languages, and traditions, which are held together in one communion by the gift of love from the Holy Spirit. The unity that Christ bestowed on his Church is something she can never lose (cf. Second Vatican Council, Decree on Ecumenism [Unitatis Redintegratio; UR], no. 4; CCC, nos. 813, 815).
Tragically, members of the Church have offended against her unity, and throughout the centuries, there have developed divisions among Christians. Already in the fifth century, doctrinal disagreements led to the separation of some Christians in the eastern region of the Roman Empire from the main body of the Church. More damaging was the rupture between Rome and Constantinople in AD 1054. And in the sixteenth century Western Europe experienced the divisions that followed the Protestant Reformation.
The Catholic Church has always been committed to the restoration of unity among all Christians. This commitment was intensified by the Second Vatican Council and led the Church to participate in what is called the ecumenical movement. The word ecumenical means “world-wide” and, in a Catholic understanding, describes efforts “for the reconciliation of all Christians in the unity of the one and only Church of Christ” (UR, no. 24; CCC, no. 822). This is to be a visible communion. “Full unity will come about when all share in the means of salvation entrusted by Christ to his Church” (Pope John Paul II, On Commitment to Ecumenism [Ut Unum Sint; UUS], no. 86). “Communion of the particular Churches with the Church of Rome, and of their Bishops with the Bishop of Rome, is—in God’s plan—an essential requisite of full and visible communion” (UUS, no. 97). Ecumenism includes efforts to pray together, joint study of the Scripture and of one another’s traditions, common action for social justice, and dialogue in which the leaders and theologians of the different churches and communities discuss in depth their doctrinal and theological positions for greater mutual understanding, and “to work for unity in truth” (UUS, nos. 18, 29). In dialogue the obligation to respect the truth is absolute. “The unity willed by God can be attained only by the adherence of all to the content of revealed faith in its entirety” (UUS, no. 18). On the worldwide level, these dialogues are sponsored on the Catholic side by the Pontifical Council for the Promotion of Christian Unity, a Vatican office directly accountable to the Pope.
The Catholic Church retains the structures of episcopal leadership and sacramental life that are the gift of Christ to his Church (cf. CCC, nos. 765, 766) and that date back to apostolic times. At the same time, the Catholic Church recognizes that the Holy Spirit uses other churches and ecclesial communities “as means of salvation, whose power derives from the fullness of grace and truth that Christ has entrusted to the Catholic Church” (CCC, no. 819; LG, no. 8). Depending on what and how much of the elements of sanctification and truth (UR, no. 3) these communities have retained, they have a certain though imperfect communion with the Catholic Church. There are also real differences. In some cases “there are very weighty differences not only of a historical, sociological, psychological and cultural character, but especially in the interpretation of revealed truth” (UR, no. 19). (The word church applies to those bodies of Christians who have a valid episcopal leadership or hierarchy, while the phrase ecclesial communities refers to those bodies of Christians that do not have an apostolic hierarchy.)
You can read more from the United States Catholic Catechism for Adults, order your own copy, or read questions about it at the United States Conference of Catholic Bishops website.
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