I beg to differ.
A religious war is a war caused by, or justified by, religious differences. It can involve one state with an established religion against another state with a different religion or a different sect within the same religion, or a religiously motivated group attempting to spread its faith by violence, or to suppress another group because of its religious beliefs or practices. The Muslim conquests, the French Wars of Religion, the Crusades, and the Reconquista are frequently cited historical examples, especially in History Books.
The Crusades were a series of military campaigns—usually sanctioned by the Papacy—that took place during the 11th through 13th centuries in response to the Muslim Conquests. Originally, the goal was to recapture Jerusalem and the Holy Land from the Muslims, and support the besieged Christian Byzantine Empire against the Muslim Seljuq expansion into Asia Minor and Europe proper. Later, Crusades were launched against other targets, either for religious reasons, such as the Albigensian Crusade, the Northern Crusades, or because of political conflict, such as the Aragonese Crusade. In 1095, at the Council of Clermont, Pope Urban II raised the level of war from bellum iustum ("just war"), to bellum sacrum (holy war).
French Wars of Religion
In 16th Century France there was a succession of wars between Roman Catholics and Protestants (Hugenots primarily). These series of wars were known as the Wars of Religion.
Thirty Years War and European wars of religion
In the first half of the 17th century, the German states, Scandinavia (Sweden, primarily) and Poland were beset by religious warfare. Roman Catholicism and Protestantism figured in the opposing sides of this conflict, though Catholic France did take the side of the Protestants but purely for political reasons.
Inspired by a formerly illegal Protestant missionary tract in China, the core of the Taiping faith focused on the belief that Shangdi, the high God of classical China, had chosen the Taiping leader, Hong Xiuquan, to establish his Heavenly Kingdom on Earth.
The Taiping rebels, professing this new creed, were able to mount their rebellion and recruit multitudes of followers in their sweep through the empire. The Taiping rebels denounced the divine pretensions of the imperial title and the sacred character of the imperial office as blasphemous usurpation of Shangdi’s title and position. In place of the imperial institution, the rebels called for a restoration of the classical system of kingship. Previous rebellions had declared their contemporary dynasties corrupt and therefore in need of revival; the Taiping, by contrast, branded the entire imperial order blasphemous and in need of replacement.
The Bible, in particular a Chinese translation of the Old Testament, profoundly influenced Hong and his followers, leading them to understand the first three of the Ten Commandments as an indictment of the imperial order. The rebels thus sought to destroy imperial culture, along with its institutions and Confucian underpinnings, all of which they regarded as blasphemous. Strongly iconoclastic, the Taiping followers smashed religious statues and imperially approved icons throughout the lands they conquered.
The Guinness Book of World Records calls this the "bloodiest civil war" with some 20 million estimated dead.
Jihad, Divisions of the world in Islam, Islamic military jurisprudence, Islam and violence, and Islamic terrorism
Jihad means "to strive or struggle" in the way of God, and is sometimes referred to as the sixth pillar of Islam, although it has no official status. In the West, jihad is often understood as Holy War. Jihads are usually called to convert other non-Muslim states. This happened many times throughout history, such as Muhammad's war against the pagan Arabs and the wars to spread Islam undertaken by the Caliphates and the Ottoman Empire.
In the Jewish religion, the expression Milkhemet Mitzvah (Hebrew: מלחמת מצווה, "commandment war") refers to a war that is both obligatory for all Jews (men and women) and limited to territory within the borders of the land of Israel. The geographical limits of Israel, and therefore of this religious war, are detailed in the Tanakh, the Hebrew Bible, especially Numbers 34:1-15 and Ezekiel 47:13-20.
In short, 809 million people have died in religious wars. That’s nearly a billion people.
Oftentimes, a retort is that secular ideals and Godless Communism have killed many more. It is true that Stalin, among others, slaughtered his own people by the millions during the industrialization of Soviet Russia. By comparison, 209 million have died in the name of Communism. Some 62 million died during World War II, civilian and military, on all sides. Conclusively, more people have died in the name of religion than in the name of Communism or Hitler, or the two combined times two.