But while spurious unity is to be condemned, the lack of unity among churches of Christ which profess the faith in its purity is a patent violation of the unity of the body of Christ, and of that unity which the prayer of our Lord requires us to promote. We cannot escape from the implications for us by resorting to the notion of the invisible church. The body of Christ is not an invisible entity, and the prayer of Jesus was directed to the end that the world might believe. The unity prayed for was one that would bear witness to the world, and therefore belonged to the realm of the observable. The implications for visible confession and witness are unavoidable.
– John Murray, “The Nature and Unity of the Church”