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