Why is Unity, Per Se, Not a Mark of the Church?
We confess that the church is one on the basis of the Apostles’ and Nicene Creeds. The reasons for this unity we explained earlier (§ 34), among which the chief is the unity of faith and doctrine (Eph. 4:5). (2) Therefore unity per se is not a mark of the church. Rather, it must be connected with faith and doctrine, Eph. 4:5: “One Lord, one faith;” v. 13: “. . . until we all attain to the unity of faith” (Athanasius, Letter ad Antioch.). “Only that is the true concord which is of faith. Without that, it is the best dissent; the most destructive concord,” as Gregory Nazianzen writes (Orat. 1, de pace).
(Johann Gerhard, Theological Commonplaces: On the Church, § 231).