Complex argument, hard to represent in a brief extract. Barth, CD I/2, p. 803:
Let us suppose the ideal case. We have a form of teaching which in its origin and first appearance, in spite of and in all its human conditioning, can only be traced back to the faithful and true hearing of the Word of God, so that what it says may be defined as the realization of pure doctrine and as the unadulterated proclamation of the Word of God. Why should we not say so at once, even if we can do so legitimately only in faith? But then the teaching goes beyond the stage of origin and first appearance. It passes from the first mouth to the second. Or it is even stated a second or third time by its first exponent. And even in the ideal case, it inevitably becomes dubious. For there is uncertainty whether this still happens in the originally faithful and true hearing of the Word of God. Perhaps the teacher concerned and the teaching church which follows him and uses his formula are already listening to themselves; to the fine and strong and timely things which they themselves have thought about the matter; to the joyful and pleasant sound which the words in question had in their mouths; to the echo of the approval and applause which they encountered; to the consequences of being publicly committed through speaking in this way to the persuasive power of a rapt congregation which wants to hear again and again what has been so wonderfully stated. Even affirmations, denials, syntheses and reservations which, as made in the original hearing of the Word, were well founded and legitimately impressive and - we will suppose - the purest doctrine, now take on a certain independence, or appear only as facile arguments, useful stones in structures with which, as originating in the Word of God, they had nothing to do, and which perhaps themselves derive from quite another source than the hearing of the Word of God. In short, the equivocal phenomenon of Church proclamation has again developed from what we will suppose was the unequivocally pure doctrine of the beginning. This development cannot be arrested. This is how it has always gone at the crises of Church history. As such, the teaching of the Church is not guaranteed even for a single moment against the possibility of immediately losing what we will suppose is its recently attained purity, and of again becoming something very different from the speech which obediently listens. A formula may be ever so simple, clear and powerful, an exposition and sequence of thoughts may be ever so transparent and compelling, but it is not guaranteed against this risk. It cannot be protected by any personality, however trustworthy, or any fellowship of believers, however faithful. Here at the very centre of its life the Church experiences the fact that it is in the world. Its proclamation begins, as we will suppose, in the Spirit, but immediately and at every point it shows a tendency to end in the flesh.
Later, on p. 807:
At every moment and in every situation the danger threatens that members of the Church may want the Word of God without God, bringing it under their power and understanding, and applying it according to their own good pleasure... No heresy has ever had the original intention of being heresy; it has become so only when and where a first unintended lapse from obedience has not been noted and resisted in time.