Why did Christians fear the advent of Islam?

Islam, at the time, was the only major religion to be seen after Christianity had grown. During a time which would lead to The Crusades, this wide-spread 'other' religion was viewed as a direct threat by the whole of Christendom.

The Islamic Caliphates were also a direct threat to the Christian Empires (like Byzantium, New Carthage, etc.) and conquered large swathes of formerly "Christian lands" in the Middle East and North Africa.