The famous confrontation of Luther and Zwingli in the castle hall of Marburg was brought about for political and personal reasons by Philip of Hesse. An account of this remarkable figure and of his client neighbour, Ulrich of Württemberg, must be deferred to a later chapter. In any case a Lutheran–Zwinglian disputation had been suggested before and had its parallel in more than one meeting with the Anabaptists. Now, even after the successful outcome of the Berne meeting, Zurich remained very much alone, able to maintain its independence but needing help, most obviously from the south German cities, for further advance.

This was realised acutely by Strassburg and by a number of its Zwinglian-inclined counterparts. In his sermon at Berne, Bucer had called for a measure of acceptance of varieties of opinion and practice among evangelicals provided the Bible remained the sole basis of faith. On this premise, neighbourly love might enhance the value of discussion and bring about the internal harmony which was his constant purpose. Differences about the Last Supper need not prevent cooperation, a sentiment repugnant to Luther, who increasingly opposed much that Zwingli stood for, doctrinally and politically. Bucer, on the other hand, pliable and plausible, with a quick and subtle brain, was indefatigable in his efforts to arrange the compromise that never came.

Jacob Sturm, Strassburg's leading politician, supported him. At the Diet of Speier (1526) he had urged on Philip of Hesse the value of the potential help available from the south German cities, and Capito also pressed for cooperation based on mutual understanding.