How did Islam impact Christianity?

Answer 1

In a religious sense, Islam did not affect Christianity, i.e. none of the Christian doctrines changed on account of Islam's existence. However, much of the Islamic World replaced former Christian countries, such as Tunisia, East Anatolia, Mesopotamia, and Spain. These conquests redirected Christian thought to reconquering and re-Christianizing those regions. There was a harsh and belligerent tone taken by Christendom to the military expansion of Islam. Christians in the Islamic Caliphates did take on different forms of dress and had a more repressed culture. The Eastern Churches had a more strained relationship with Rome which endures to this day.