The Crusades in Europe: The History of the Catholic Crusaders’ Campaigns against the Ottomans and Christians across the Continent

Christianity was not a state religion for its first three centuries, and it was only when Emperor Constantine the Great declared it so in the early 4th century that the Church was faced with the thorny problem of state-sanctioned violence. The first major Christian authority to justify the use of arms in defense of Church and State was Augustine, Bishop of Hippo, who wrote in the 5th century, “They who have waged war in obedience to the divine command or in conformity with His laws, have represented in their persons the public justice or the wisdom of government, and in this capacity have put to death wicked men; such persons have by no means violated the commandment, ‘Thou shalt not kill.’”

This opinion gained increasing influence in Western Christianity, though in the East, the attitude was (and continues to be) more nuanced. War was tolerated as a regrettable necessity in a world wounded by sin but never blessed. Canon law enacted in the Byzantine (Eastern Roman) Empire tended to treat soldiers who had killed as sinners needing to repent, and Bishop Basil of Caesarea (d. c. 330) believed that they needed to abstain from receiving communion for three years after battle. It was not that that the Eastern Roman Empire was a particularly peaceable state–far from it, in fact: it was engaged in almost continuous warfare for its entire existence. However, its conflicts were mostly defensive in character, fighting barbarians, Persians, or Muslims, and the idea of consecrating arms for the cause of Christianity was considered alien to its spirit.

At the Council of Clermont in July 1095, Pope Urban II canonized religious war by urging Western Europe's nobility to take up arms in defense of the Byzantine Empire against the Muslims, thus launching the Crusades, and religious military orders such as the Knights of Saint John, the Templars, and the Hospitallers arose.