Perhaps the two biggest factors that led to a decline in violence among Christian groups were the creation of the modern nation state and the development of the idea of religious toleration in the 17th century. The Peace of Westphalia in 1648 is usually seen as a pivotal moment in ending the wars of religion. It cemented the idea of nation-states as sovereign territories, where other Christian states weren’t supposed to interfere with their neighbors due to religious differences. It also provided limited religious toleration among Christian groups in the Holy Roman Empire.

In the same time period a number of philosophers were articulating the idea that states should allow religious believers to be unmolested in their faith. Baruch Spinoza’s 1670 Tractatus Theologico-Politicus argues for this kind of religious toleration. John Locke’s appropriately named “Letter on Toleration” in 1689 argues that the state should not use force to compel religious belief. Some religious thinkers thought along the same lines. The founder of what would become the Colony of Rhode Island, the minister Roger Williams, wrote a justification for religious toleration based on theological grounds in 1644.

The acceptance of these ideas about religious toleration was not uniform or instant. For example, in England, the Toleration Act of 1689 permitted Protestants outside the established Anglican Church to worship, but legal restrictions on Unitarianism were only removed in the nineteenth century. Roman Catholic France actually became less tolerant to Protestants in the late seventeenth century, after it revoked the Edict of Nantes.

In the United States there was widespread legal toleration of religion, but mass violence was known to occur between Catholics and Protestants well into the nineteenth century. In 1834 a Massachusetts mob made up of Protestants burned down a convent. The 1844 Philadelphia “Bible Riots” were caused by Protestant fears that Catholics would restrict Bible reading in schools and killed over a dozen people. Such violence declined after the Civil War.

Overall, the trend of making religion a private matter -rather than a political one- probably reduced inter-Christian violence. It wasn’t necessarily a uniformly positive development though, a few scholars make the interesting argument that conflicts between states become more brutal as a result.