Konrad Grebel published a snippet contra Zwingli and his position on baptism on the 7th of December, 1524. On the 28th of the same month Zwingli let fly his own ‘Pearl Harbor attack’ on the ‘theology’ of Grebel in his Wer Ursache gebe zu Aufruhr usw. Actually, the full title is Welche ursach gebind ze ufruoren. Welches die waren ufruorer sygind. Und wie man zuo cristlicher einigheit und fryden kommen möge. Durch Huldrych Zuinglin zuo Zürich, etc.
Zwingli delivers a lengthy full frontal attack not only on Grebel’s views but on the stance of the troublemakers as a whole. The opening lines tell the story-
Aller liebsten brueder in gott! Es ist nieman unerkant, wie vil widerstands und muesal erlyden muessend alle, die gottes wort anhangen wöllend, als ouch Paulus 2. Timmo. 3. [2. Tim. 3. 12] anzeygt. Daruß wol ze vermessen ist, das ouch ir vil verspottung, lestrung, tratz und tröuwen tragen muessend darumb, das ir in erkantnus der warheyt kommen, und die angenommen habend. Doch sol üch sölicher widerstand gheinen weg krencken, sonder ye me und me im glouben stercken; dann wir sicherlich an widerwertigheiten erlernend, was recht gegloubt, recht gebättet, recht geraten, recht gethon sye.
Brilliantly Zwingli suggests that the Grebel-ites are in fact persecutors of those who preach the truth and who are spreading the Gospel in its fullness.
Of Grebel himself, Simpson remarks
Conrad Grebel [was] a young man descended from one of the best families of Zurich, brother-in-law of Vadian, and a former friend of Zwingli. He was a man of fine scholarship, having studied at the universities of Paris and Vienna; morally, however, his career had been anything but creditable. At school he had led a life of such wild dissipation as to ruin his health and squander a considerable fortune.*
That’s putting it mildly. Grebel was of such a character that while in Paris he picked up a nasty case of syphilis, which affected his mind (as syphilis is inclined to do) to such a degree that he became more and more unstable- as his interactions with Zwingli show. The sadly ironic thing is, though, that it was Zwingli who helped Grebel and led him to turn from his life of licentiousness. Grebel was, to Zwingli, little more than a traitor to the Reformation.
*S. Simpson, Life of Ulrich Zwingli: The Swiss Patriot and Reformer, p. 147.