Prove it.
The greatest cause of World War I was the assassination of Archduke Franz Ferdinand of the Austro-Hungarian Empire. He was assassinated by a member of the Black Hand named Gavrilo Princip. Princip was a Bosnian Serb and a Yugoslavian nationalist. Any other plausible causes are of politics, not religion, such as the Balkan Wars, the first of which was fought when the Balkan League attacked the Ottoman Empire, ending its 500-year ruling in the Balkans. Overall, imperialism, nationalism, militarism, and alliances were the largest contributors to the start of the First World War. Please know your facts before saying things that are false.