I’m familiar with the history, and there have been many assassinations, but the article does not say why this particular assassination is the end of European Christendom.
Well, it triggered WWI which effectively started the decline of European imperialism, and also paved the way for the rise of Fascism, Communism, and Socialism in Europe. Those atheist systems have now replaced the old republics and monarchies in every major nation in Europe.
I had the same question.
I surmise he means that as absolute monarchs gave way to Democracy in Europe, that the death rattles of authoritarianism that spawned two world wars and the cold war, too, pitting authoritarian model governments, though not literal monarchies, against Democracy.
The author believes that authoritarianism embodied in monarchy is the essence of Christianity and that once commoners are allowed to make decisions upon reason rather than upon blind acceptance of faith, that Christianity is undermined. He may be right.