How do the crusades show the problems with when two universalizing religions meet?

A universalizing religion is a religion that is not specifically related to one ethnicity (Hinduism, Judaism) but a religion that intends to convert everyone to it - so an universalising religion would not coexist in peace with other religions, but fight them for the followers.

Crusades happened because the followers of Christianity tried converting the Muslim, and those did not want to, and instead would have wanted to convert the Christians to their religion.