After the death of Christ, Peter, his chief apostle, became the Bishop of Rome and is considered the first pope in a line of succession by which the bishops of Rome became the papal rulers of the Church. The apostles, incidentally, were married, as were many of the early popes. By the end of the first millennium, the Papal Reform Movement, spearheaded by Pope Leo IX (1049-54), sought to reform the priesthood by eliminating marriage for priests. Around the same time, the Crusades began. The Holy Land of Jerusalem, a pilgrimage site for Christians, had been taken over by Muslims in 637, and they eventually restricted access by non-Muslims.
St. Augustine’s concept of the “just war” held that war was justifiable if it was waged at the implicit command of God, and subsequent popes developed the notion that death in battle for the sake of the Church would result in heavenly rewards similar to those for martyrdom — a concept shared by Muslims today. These beliefs created a fertile theological ground for the Crusades. The large armed escorts sent to accompany pilgrims to the Holy Land with Muslims approval often ended up murdering Muslims, Jews, and even other Christians, and engaged in looting and gratuitous destruction. The First Crusade was set in motion by Pope Urban II in 1095; the crusaders set out and took Jerusalem in 1099. The Second Crusade in 1147, led by King Louis VII of France and Holy Roman Emperor Conrad III, was a failure. Forty years later, Saladin took Jerusalem back from the Christians, instigating the Third Crusade by the three principal monarchs of Europe, Including Richard the Lionhearted. They failed to capture Jerusalem, but worked out a truce with Saladin allowing passage to the Holy Land. The Fourth Crusade was carried out not against Muslims but the Eastern Orthodox of Constantinople in 1204. It succeeded in permanently alienating the Greek and other Orthodox from the Roman Church.
These failures led many Christians to think that the Crusaders were not innocent enough. So in 1212, when a French peasant boy preached a Children’s Crusade, 50,000 French and German youths set out on one of the great follies of the Middle Ages. Most of them either perished while crossing the Alps or were sold into slavery in Marseilles. There were eight Crusades in all, most of them failures except for the stimulus they provided to European commerce and the arts as a result of extended interaction with the East.