In 313 CE, the emperor Constantine issued the Edict of Milan, which granted Christianity—as well as most other religions—legal status. ... In 380 CE, the emperor Theodosius issued the Edict of Thessalonica, which made Christianity, specifically Nicene Christianity, the official religion of the Roman Empire.
There are several ways in which nationalism, imperialism, and militarism helped set the stage for World War I, but it general it made each European country feel that it was superior to the others.