The classical theological view has always been that unity, holiness, catholicity and apostolicity are the "four marks" of the Christian church. Various evangelicals attempt to add "other" marks to these, some even popularly speaking of "nine marks of the church." I have no dispute with this notion in terms of a general consideration of the work of the church but there are not "nine" marks of the church in the confessional and historical sense. The danger comes when "nine marks," or any other number we come up with on our own, becomes a new form of sectarianism, adding to the confessional life of the church a list of items that a few individuals believe are central to faith and practice when the church catholic has deemed otherwise.
The view that there are four marks grows out of a deep understanding of the Nicene Creed. The creed confesses faith in the church as "one, holy, catholic and apostolic." Contemporary theologian Jürgen Moltmann writes of these four marks as integrated confessional components of the triune God, thus they must be understood (he believes) as statements of faith, hope and action (love). For Moltmann these marks are not so much characteristics that the church possesses as they are characteristics of Christ's own activity, thus statements of faith on our part. What Moltmann means is this—unity, holiness, catholicity and the apostolic character of the church are all statements about what Christ is presently doing; in, with, and through the church. At the same time these are statements about "Christ's messianic mission and the eschatological gift of the Spirit" therefore they are "messianic predicates of the church in the perspective of his coming kingdom, for which it exists and which in the church acquires form and testimony."
Here is the critical point in Moltmann's theology regarding the four marks of the church: Whenever we speak of any one of these marks, or all four of them together, we are making statements of hope, not statements about full possession. But as statements of biblical hope, and this is extremely critical to grasp, these are also statements of faith. And as statements of faith they are statements of action. As a statement of faith these four marks remind us of Christ's work first and foremost. As statements of hope they remind us that the kingdom of God will come on earth as it is now in heaven. But faith and hope always lead us to action, thus to love. These marks always call us to be one, to be holy, to be catholic, and to be apostolic. Taken together, in this kind of rich theological understanding, you can see why these are the real marks of the church. They make visible and present the real life of the church, that life which cannot be replicated through teaching various other marks that we come up with on our own.
Moltmann's comments about this subject are worth a more careful reading:
The church's essential nature is given, promised and laid upon in the characteristics. Faith, hope and action are the genius of the form of the church visible to the world in unity, holiness, catholicity and apostolicity. That is why theology cannot withdraw to the "the invisible church," "the church of the future," or "the church of pure demands." The church lives in the one, holy, catholic and apostolic rule of Christ through faith, hope and action (Church in the Power, 340).
This theology offers us a helpful, and I would submit practical, way of understanding the church. There is a critical tension within the real church, a tension with what we are and how we are to live faithfully. We cannot escape this tension. This tension calls for self-critical reflection, constant discernment and the ongoing renewal of the Spirit. This is where the oft debated term reformata semper reformanda (the church is "reformed [and] always reforming) comes in, at least in spirit. This tension is so serious and powerful that we are continually called to pursue total renewal. It will involve, according to Moltmann's paradigm, facing resistance from without the church and also from within.
This also raises questions that address ongoing conflicts within a particular society. In South Africa this was the case with apartheid between 1948 and the election of Nelson Mandela and modern reform in the 1990s. This challenge produced a confession from within the church called The Belhar Confession. This confession is now in the process of being adopted by my own denomination, the Reformed Church in America. If this adoption finally takes place Belhar will have reached far beyond its original context to speak to the racial (and other) divisions that exist in our own American churches. What fascinates me is how people on the right "fear" Belhar because they see it as a way to embrace the homosexual agenda inside the church. I do not doubt that this could happen but I will not decide my own view on such a subject based on what "could" happen when I am seeking to correct a problem or address a present need.
The church in South Africa was served by neither the ruthless pluralism of apartheid nor by attempts to establish rigid uniformity. Practicing unity in freedom is vital to the health of the church. But this is where the struggle really comes. This is why the struggle over sexuality is so difficult in the modern West. I remain strongly opposed to adopting sexual standards that oppose the commandments of God while at the same time I work against rigid uniformity as the response of so many Christians. This means that I am always in a place where tension over such issues is quite real. The easy way to deal with such tension is to simply walk away and divide the church again. But sometimes division will (must) come. In this case that seems to be true in the Episcopal Church U.S. (I have more than a few friends who remain in a particular local parish out of conscience even though they strongly repudiate the actions of the ECUSA as a denomination.) My central point here is not to sort out the present tensions with simple solutions. Actually, my point is quite the opposite. This is true because I believe this dynamic tension will always call us to continuing reformation. This, I believe is desperately needed in the North American church. I thus believe such tension is integral to Christian faithfulness and to the history of faith among all who confess the triune God. But if you fail to confess these four marks you will be very likely to move either toward ruthless pluralism, as is happening in some liberal/progressive American Protestantism, or rigid uniformity, which very often happens in rigid conservatism. I believe it is time that ordinary, faithful and serious Christians understood these four marks in this way and then took seriously their contribution to the work of reformata semper reformanda. This will require deep faith, real hope and earnest action (love).