In the spring of 1529 Lutherans joined Catholics at the Diet of Spires in refusing toleration to the Zwinglians.

The division of Protestants of course weakened them.

Their leading statesman, Philip, Landgrave of Hesse, seeing this, did his best to reconcile the leaders.

For several years he tried to get them to hold a conference, but in vain.

Finally, he succeeded in bringing together at his castle at Marburg on the Lahn, Luther, Melanchthon, Zwingli, Oecolampadius, and a large number of other divines.