Quoting Dr. Francis Pieper:
With regard to the orthodox character of a church body note well: (1) A church body is orthodox only if the true doctrine, as we have it in the Augsburg Confession and the other Lutheran Symbols, is actually taught in its pulpits and its publications and not merely “officially” professed as its faith. Not the “official” doctrine, but the actual teaching determines the character of a church body, because Christ enjoins that all things whatsoever He has commanded His disciples should actually be taught and not merely acknowledged in an “official document” as the correct doctrine. It is patent that faith in Christ will be created and preserved through the pure Gospel only when that Gospel is really proclaimed. (2) A church body does not forfeit its orthodox character by reason of the casual intrusion of false doctrine. The thing which the Apostle Paul told the elders of Ephesus: “Also of your own selves shall men arise speaking perverse things to draw away disciples after them” (Acts 20:30), came true not only in the Apostolic Church, but also in the Church of the Reformation and will occur in the Church to the Last Day. a church body loses its orthodoxy only when it no longer applies Rom. 16:17, hence does not combat and eventually remove the false doctrine, but tolerates it without reproof and thus actually grants it equal right with the truth.