When we say that the pure ministry of the word and pure celebration of the sacraments is a fit pledge and earnest, so that we may safely recognise a church in every society in which both exist, our meaning is, that we are never to discard it so long as these remain, though it may otherwise teem with numerous faults. Nay, even in the administration of word and sacraments defects may creep in which ought not to alienate us from its communion. For all the heads of true doctrine are not in the same position. Some are so necessary to be known, that all must hold them to be fixed and undoubted as the proper essentials of religion: for instance, that God is one, that Christ is God, and the Son of God, that our salvation depends on the mercy of God, and the like. (Institutes 4.1.12)
What else is there is to add?