This gospel text allows of no other doctrine beside it; it desires only to testify of Christ and lead men to him, who is the Light. Therefore, O Lord God, these words, "He was not the Light," are truly worthy to be capitalized and to be well remembered against the men who set themselves up as the light and give to men doctrines and laws of their own fabrication. They pretend to enlighten men, but lead them with themselves into the depths of hell; for they do not teach faith, and are not willing to teach it; and no one teaches it except John, who is sent of God, and the holy Gospel. Truly much could be said on this point.
In short, he who does not preach the Gospel to you, reject and refuse to hear him. He, however, preaches the Gospel who teaches you to believe and trust in Christ, the eternal Light, and not to build on any of your own works. Therefore beware of everything told you that does not agree with the Gospel; do not put your trust in it, nor accept it as something external, as you regard eating and drinking, which are necessary for your body, and which you may use at your pleasure or at the pleasure of another; but by no means as something necessary to your salvation. For this purpose nothing is necessary or of use to you except this Light.
O, these abominable doctrines of men, which are now so prevalent and which have almost banished this Light! They all wish to be this light themselves, but not to be witnesses of it. They advocate themselves and teach their own fancies, but are silent about this Light, or teach it in a way as to preach themselves along with it. This is worse than to be entirely silent; for by such teaching they make Samaritans who partly worship God and partly worship idols, 2 Kings 17,33.