In the spring of 1214 he began to preach the crusade in the Rhineland, the Netherlands, and Friesland, where he succeeded in recruiting thousands of volunteers who pledged to take up the cross.
In August 1218 the Friesian crusaders distinguished themselves by their success in destroying Damietta's upstream tower, located in the middle of the Nile.
Oliver wrote in his Historia Damiatina that in 1219: "Before the capture of Damietta there came to our attention a book written in Arabic, in which the author says that he was neither Jew nor Christian nor Saracen."
Also "that a certain king of the Christian Nubians was to destroy the city of Mecca and cast out the scattered bones of Mohammed, the false prophet, and certain other things which have not yet come to pass."
According to Ahmed Sheir: [3] This anonymous letter mentioned that the son of Prester John, King David, sent his envoys to release the Christian captives captured in Egypt during the Crusaders’ siege of Damietta, and then they were sent to the Abbasid Caliph in Baghdad as gifts.This was the first mention of a King David as a descendant of the legendary Nestorian ruler who was going to join with Western Christendom in a combined effort to destroy Islam.
The King David story soon grew to such proportions and generated so much excitement among the crusaders that it led them to prematurely launch an attack on Cairo during the Nile flood season, resulting in their complete defeat at the hands of the Muslims.
The choice wasn't undisputed, however, and the opposing candidate, Heinrich von Brakel, provost of Busdorf, received the bishop regalia from the king and confirmation from the archbishop of Mainz.