Ministers of the Word, therefore, if they would be counted faithful and prudent on the Day of Christ, out to be very sure that St. Paul did not speak empty words or prophesy of a thing of nought, when he said: “There must be heresies among you, that they which are approved may be made manifest among you.” Let the minister of Christ know, I say, that as long as he preaches Christ purely, there will be no lack of perverse persons, even among our own people, who will make it their business to cause trouble in the Church. And he may comfort himself with the thought that there is no peace between Christ and Belial, or between the Seed of the woman and the seed of the Serpent. Indeed, he may rejoice in the trouble he is caused by sects and the constant succession of seditious spirits. For this is our glory, the testimony of our conscience that we are found standing and fighting on the side of the Seed of the woman against the seed of the Serpent. Let him bite our heel and never cease biting; we for our part will not cease to crush his head through Christ, the first to crush it, who is blessed for ever. Amen.

Martin Luther in his introduction to his Commentary on Galatians