From Real Clear History by Brandon Christensen:
World War I is responsible for destroying Christianity as a moral order. Christianity survives today, of course, and even thrives in parts of the world, but it does so in the West as a form of resistance or as a reprieve from the day-to-day grind of life in secular democracies. This was not always the case. What is now known as Europe was once referred to as “Christendom” due to the fact that Europeans by and large operated under a Christian moral order.
This is a tough sell, but look at the Middle East. Today, the Middle East is often referred to as the “Muslim World” and Middle Eastern states are commonly known as Muslim states. Prior to World War I, this was also the case with European countries. This understanding, of Europe as Christendom, became weaker as the 18th and 19th centuries progressed, but large swaths of the world still thought of Europe as Christendom and many foreign affairs conducted by European governments were viewed through the lens of Christianity up until the end of World War I. Christianity enjoyed a cultural prominence in European societies, even the secular ones, that controlled the moral order of European thought and action. Christianity was hegemonic in Europe.
There were intra-European wars, such as the 30 Years’ War, but when the time came to cease hostilities, diplomatic entreaties were made in the form of Christian brotherhood rather than as sovereign states in a world order. This all changed with World War I. The savage slaughter of souls on all fronts wrought the death knell of Christianity in the West.