That is a good question that I as a Rabbi have often been asked.
This is my answer. The Qur’an declares that Allah could have made all of us monotheists, a single religious community, but didn’t in order to test our commitment to the religion that each of us have been given by God.
“If Allah had so willed, He would have made you a single people, but (God’s plan is) to test you in what He has given you: so compete in all virtues as in a race. The goal of you all is to (please) Allah who will show you on the truth of the matters in which you dispute.” (Qur’an 5:48)
This means that religious pluralism is the will of God. Yet for centuries many believers in one God have chided and depreciated each other’s religions, and some believers have even resorted to forced conversions, expulsions and inquisitions. Monotheists all pray to the same God, and all prophets of monotheistic faiths are inspired by the same God.
So how did this intolerance come about, and how can we eliminate religious intolerance from the Abrahamic religions. Greek philosophy, with its requirement that truth must be unchanging and universal, influenced most teachers of sacred scripture during Medieval times to believe that religion was a zero sum game; the more truth I find in your scripture the less truth there is in mine.
Instead of understanding differing texts as complementary, they made them contradictory and declared the other religion’s sacred text to be false.
In 1065 Pope Alexander called for a Crusade against the Moors in Spain. Then in 1073, Pope Gregory issued official proclamations urging Christian princes to recover lands occupied by Muslims in Spain, over which he claimed papal sovereignty on the basis of previous Christian rule and right. These decrees were really the beginning of the Christian Crusades against Islam.