
B. Every believer for the sake of his salvation must flee all false teachers and avoid all heterodox congregations or sects.
Many, on hearing that the church exists wherever the Word and the sacraments are still found essentially, infer from this fact that it is a matter of indifference whether they belong to an orthodox [rechtglaeube] or to an unorthodox [falschglaeube] church, since after all they are in the church and so can be saved. But they are mistaken. True, it is not necessary to leave a heterodox communion in order to be in the church, and many are indeed saved who, for lack of knowledge, outwardly belong to sects and nevertheless continue in the [true] faith. But what does it profit anyone to be in the church if he is not of the church and [so] does not belong to it? Whoever has learned to know the false doctrine of the sects and their teachers and despite this fact continues to belong to them is indeed still in the church but not of the church. Such a person does not belong to the divine seed that is hidden in the sects. His communion with the sects is not a sin of weakness, with which the state of grace can exist, for such a person acts willfully and contrary to the will of God, who in His holy Word commands us to flee and avoid false teachers and their false worship.
As little, therefore, as the doctrine that true [begnadigte] Christians still commit sins of weakness justifies those who think that for that reason they knowingly and willfully may continue in sin, indeed, as surely as those who thus sin against [divine] grace are children of perdition, so little also does the doctrine that in the sects there are children of God justify those who contrary to God's Word knowingly desire to continue in them; indeed, so surely also such willful partakers of the perversion of the Word of truth are children of perdition...
[p.114 C.F.W. Walther. Church and Ministry (Kirche und Amt) trans. J.T. Mueller (Saint Louis: Concordia Publishing House, 1987]






