The Catechism in Questions & Answers

594. What else did the Christians have to contend with starting in the seventh century?

Starting in the seventh century, Christians in parts of Asia, Africa, and even Europe had to contend with a new religion, namely Islam. Many areas were lost to the Christian faith, for example the Middle East and Northern Africa.
This led to battles, for example, the Crusades. These took place between 1095 and 1270 in the Middle East with the stated objective of conquering Jerusalem and the Holy Land for Christendom.

Islam is the youngest of the major world religions. It was established by Mohammed in the seventh century AD. Islam teaches belief in one God, but not in a triune God. In Islamic teaching, Jesus is considered a prophet. The holy book of Islam is the Koran.
Crusades: Palestine, and with it Jerusalem, were under Islamic rule. Between the eleventh and thirteenth centuries, successive popes called upon the rulers of Europe to bring this region back under Christian control. These military campaigns were called “Crusades” and its soldiers were known as “Crusaders” because they went to battle in the name of Christ and for His glory.