There is one ground for the denial of church fellowship about which there was never anywhere a difference of opinion in the early church, not even between East and West. Heterodoxy breaks the fellowship ipso facto. . . . What is true [when considering the local congregation] is true also between churches.

The divisive significance of dogma is only one side of the matter. Dogma is not only the binding doctrinal norm for those who teach in the church, but it is also the confession of all the members who are included in the “We confess” or “We believe” [of the Creed]. For this reason doctrine is the point at which the unity of the church is most grievously wounded and therefore the point at which also the wounds must again be healed. Where church fellowship is broken by heterodoxy, it can only be restored by the achievement of doctrinal unity. Doctrinal unity is part and parcel of orthodoxy. The truly sound faith leads “to fellowship and unity with those who believe the same.”

Until the opposite is proven, what a man says is accepted as that which he really thinks. In the excommunication of the heresiarchs all who think “his thoughts” or “the same” are also included. Similarly the heirs of the Nicene Fathers “who proclaimed the same kerygma” are at one in their “thoughts of the faith.” Threatening divisions are to be avoided by a renewal of the Nicene faith so that thus those “who think the same would be led to unity.” . . .

Doctrinal unity means accord in the “understanding of the faith.” This may not be subjectively misunderstood. Dogma is an expression of the faith, a confession of what is believed. It is not the personal act of believing that forms the unity but what is believed.

This is true for all Christians. It is true in a special way for bishops. Unless he is contradicted, a bishop may regard himself as united in the faith with his own congregation. Since he has the office of teacher, he represents what is taught both within and without. For this he does not have to be a professor. Dogma is the basic stuff of the whole divine service. He is thus a constant defender against heresy. As soon as some alien doctrine arises, he is drawn into controversy by virtue of his office. . . .

As theological questions arose [in the history of the church], they became the common concern of the bishops. In their letters to one another the voice of each had its place in asking questions and offering answers. We see how doctrinal unity is always in question. It is not an assured possession but an unremitting task. Since doctrinal unity is always corporate, it can only be maintained jointly.

(Werner Elert, Eucharist and Church Fellowship in the First Four Centuries, translated by Norman E. Nagel, Concordia Publishing House, 1966, pages 143-144)