The purpose of the Crusades was for Christians in Europe to retake Jerusalem in the name of Christianity from the Jews and Muslims. In fact, Pope Urban II told those Christians who were to fight that if they died during the Crusades, they would have their sins forgiven.
Before those in the first Crusade reached Jerusalem, they had already spent dozens of months fighting their way across Europe. By the time they arrived in Jerusalem, the Muslims and the Jews had been warned and were prepared to fight. As the crusaders entered into the city, Jews and Muslims fought together to defend themselves. When they sought refuge in synagogues, crusaders set many of these edifices on fire. Many of those Muslims and Jews who did not die in the fighting were sold into slavery. The Muslim and Jewish community of Jerusalem was greatly diminished for many years.