The correct answer to this open question is the following.
European Christians' view of infidels led to the crusades in that they had to maintain intact the principles of Christianity as were taught by the Bible and had to defend their sacred territory from the invasion of the Muslim people.
European Christians wanted to force other people to believe in what they believed. They wanted to impose their criteria on Christian orthodox, Muslims, and Jews. But wneh Muslims captured Jerusalem and stated that the Temple of Solomon was their territory, European Christians sent troops to the Middle East to defend what they considered belonged to them.