The Pope.
Pope Innocent III
The Pope asked them to fight and many Christians answered his call.
No, he sent Christians to fight. The Turks were Muslims.
The Byzantine Emperor (Alexius) asked Pope Urban for Christian knights to help him fight the Muslims Turks.
True
European knights were urged to join the Crusades by Pope Urban II. They were to rout the Muslims and free the holy city of Jerusalem as well as protect the Holy Roman Empire at Byzantium in 1095. The reward was forgiveness for all of their sins.
The Crusades were launched by Pope Urban II, not Pope Urban III.
The pope told them to and they thought they were fighting for God's holy city!
Pope Urban demanded that all Christians were to reclaim the holy land from the Muslims.
A Crusade was made so that the they could get land from the Muslims control. The knights were called from the Pope Urban II because they were told it was a religious duty
It was a famous quote from Pope Urban II's speech to go off to fight in the Crusades. He said "Deus vult!", meaning "God wills it!", referring to the Crusades.