The Nature of the Church
Cardinal Ratzinger, New Outpourings of the Spirit
St. Thomas Aquinas, while describing the Church as the Mystical Body, explained its nature in various ways. All members of the human race on earth belong to the Mystical Body potentially, he wrote, but not actually, for those who are not members of the Church merely have the capacity to become members. In the broadest meaning of the Mystical Body, all the saints of the Old and New Testaments belong to the Church Triumphant in heaven, even though the patriarchs of the Old Testament lived before Our Lord founded His Church on the Apostle St. Peter.
In the centuries after St. Thomas a new problem arose, when the Protestants preached an invisible Church of saints in opposition to the visible Catholic Church on earth. The Anglican form of this teaching was the "branch theory," which made Catholics, the Orthodox and the Anglicans all different "branches" of the invisible Church taught by Protestantism. ...the only true "branch" theory is that of the Gospel of St. John 15:6. There is only one Vine, and branches must be united to this one Vine to have life. If there is confusion about the nature of the Church after the Second Vatican Council, Catholics must apply Pope Benedict XVI’s hermeneutics of continuity by turning to the teaching of prior popes such as Leo XIII and Pius XII, who taught how the only living vine is the Mystical Body which is the Catholic Church, the Soul of which is the Third Person of the Blessed Trinity. A half century after the start of the Council and the resulting ecumenism, we must always remember that the Catholic Church alone is the one true Church of Our Lord Jesus Christ.
All Christians are members of ONE Body of Christ. We recognize the validity of their baptism. But they are not in full communion in the Body of Christ as long as they reject some elements of the Catholic faith. This means that the Body of Christ continues to be broken and wounded. This also means that we have a responsibility to witness and teach the fullness of the Catholic faith.