The Creed of the Council of Constantinople (381), better known as the Nicene Creed, contains the line: "We believe in one holy catholic and apostolic Church." These are known as the marks, or notes, of the Church.

They were used by Counter-Reformation theologians to distinguish the Catholic Church as "the one, true Church of Christ" from all of the "false" claimants that had emerged from the Protestant Reformation.

The Catholic apologists argued that the true Church must possess the marks of unity, holiness, catholicity and apostolicity, as specified in the Creed, and that the Catholic Church alone has them in a visible, easily verifiable form.

The best English-language treatment of the marks of the Church is Francis Sullivan's "The Church We Believe In: One, Holy, Catholic and Apostolic" (Paulist Press, 1988). The Jesuit theologian has a highly compressed version of the book in the one-volume "HarperCollins Encyclopedia of Catholicism" ("marks of the Church," pp. 817-18).

As Father Sullivan points out therein, the apologetical approach of the Counter-Reformation period was severely marred by the theological presuppositions that were prevalent at the time. The four marks of the Church, he observes, "were practically reduced to the requirement that the true Church must be one governed by the Bishop of Rome."

Another problem with the apologetical approach was its insistence on the visibility of the marks, such that any objective person could readily see them in the Catholic Church. But, as Father Sullivan insists, "there is much more to the oneness, holiness, catholicity, and apostolicity of the Church than what can be seen of them."

Catholic theologians today, and since the mid-20th century, have abandoned the apologetical approach in favor of one that is called eschatological. This means that the four marks are as much future goals of the Church, still to be achieved, as they are present realities.

Thus, the Church already experiences some measure of unity because of the presence and activity of the Holy Spirit, who is the principal source of unity, and also because of the Eucharist, which is the sacrament of unity.

The oneness of the Church is also, but not primarily, insured by the Petrine ministry of the Bishop of Rome, the pope, whose major pastoral responsibility is to insure the unity of the whole Church.

At the same time, however, the Church is not united; it is divided: East from West, and Protestant and Anglican from Catholic. So it is theologically --- and empirically --- more accurate to say that the Church is already one, but not yet fully one.

And that is what is meant by the term "eschatological." The Church already shares in the unity of the triune God by the working of the Holy Spirit, but at this moment of salvation history it "strains toward the consummation of the kingdom and, with all its strength, hopes and desires to be united in glory with its King" (Dogmatic Constitution on the Church, n. 5).

The same can be said of the other marks. The Church is already holy, but not yet fully holy. The Church is already catholic, but not yet fully catholic. The Church is already apostolic, but not yet fully apostolic.

The fundamental source of the Church's earthly unity, as noted above, is the Holy Spirit, who is available to all Christians, Catholic and non-Catholic alike.

The next most powerful source of unity is the Eucharist, in the celebration of which the Church prays for, and to some significant extent receives, the grace of God that creates and sustains the unity of the whole, yet still-divided, Church.

For many Protestant communities, however, the liturgical emphasis is on the preached Word to the practical exclusion of the sacrament. Consequently, the Eucharist plays a more limited role in sustaining and strengthening the unity of the earthly Church than does the Holy Spirit.

And the Petrine ministry, exercised by the pope, has an even more limited role, because fewer portions of the Body of Christ recognize his primacy over the whole Church than celebrate the Eucharist or some form thereof.

To be sure, the Catholic Church teaches that it is not simply one Church among many. It is, in some real theological sense, the one, true Church of Christ in which the Body of Christ "subsists," even if not fully so (Dogmatic Constitution on the Church, n. 8).

But Vatican II also acknowledges that there are other Churches and "ecclesial communities" within the Body of Christ, which share faith in Christ, celebrate one Baptism, and are committed to the Word of God and its moral demands (Decree on Ecumenism, n. 3).

The Church is one, but not yet one.

Father Richard P. McBrien is the Crowley-O'Brien Professor of Theology at the University of Notre Dame.