It was in the thirteenth century that Western Christendom began to be shaken in the conviction that it possessed the absolute truth. The Church that had converted pagans, excommunicated heretics, and excluded the Jew from the common life, now found herself face to face with Islam. That enigmatic monarch Frederick II entered Jerusalem by treaty with the Sultan of Cairo and in defiance of the Pope, and the dwindling Christian principalities of Palestine took advantage of a truce with the enemy to turn against each other. Franciscans and Dominicans worked in North Africa and the Near East, sometimes meeting with martyrdom as the reward of their zeal. But the missionary needs to know the language of those to whom he goes. One of the most enterprising missionaries of the period was the Franciscan, William Rubruck, who entered the Mongol Empire and spent some time at the court of the Great Khan.