From the beginning, though Luther and Zwingli were at one in their opposition to Rome, they were unable to agree upon a common religious platform. The Sacramentarian controversy, confined at first to Luther and Carlstadt, grew more embittered after Zwingli had espoused openly the side of the latter. Several German princes having embraced the views of Zwingli, it was felt necessary to preserve some kind of unity amongst the Reformers, especially in view of the threatening attitude assumed by Charles V. A conference was called at Marburg (1529), at which Luther, Melanchthon, Osiander, and Agricola agreed to meet Zwingli, Oecolampadius, Butzer, and the other Swiss leaders. The conference failed to arrive at a satisfactory agreement, but in 1536 the Concord of Wittenberg was concluded, whereby it was hoped that peace might be restored by the adoption of a very ambiguous formula. Luther, however, refused to allow himself to be bound by the agreement, and the controversy went on as violently as before. In the meantime Calvin had undertaken to preach doctrines on the Eucharist entirely different from those put forward by either Zwingli or Luther, with the result that Zurich found itself in conflict with Geneva as it had found itself previously in conflict with Wittenberg. To restore some semblance of unity among the Swiss Reformers Bullinger, the recognised head of the Zurich party, entered into communication with Calvin, and a doctrinal agreement was arrived at known as the /Consensus Tigurinus/ (The Zurich Concord) in 1549. Later on this was confirmed by the /Confessio Helvetica/ (1564).