If you can't be bothered reading my autistic rambling: World War I.

The Protestant Reformation was generally the beginning, with the prior Renaissance arguably being the clearest "peak" of the West in terms of its cultural and societal development. From the Protestant Reformation, Anglo-liberalism formed and provided the strongest groundworks for what would become modern liberalism albeit failing to spread itself significantly outside of the Anglosphere (England, Scotland and the Thirteen Colonies). The French Revolution was when the ball of decline began to slowly roll down the steep, the first implementation of what could be seen as modern liberalism and its subsequent forceful spread across Europe through the French Revolutionary Wars. It saw the initial emancipation and normalisation of the Jews in society (obviously this was not instantly, but the Napoleonic puppet states were the first to do so), set liberalism (alongside Anglo-liberalism) as the norm which coupled with the Industrial Revolution and the fall of feudalism allowed the rise of the bourgeois as the dominant ruling class and the Jacobins provided the inspiration for what would become modern socialism from the likes of Fourier, Proudhon, the Paris Commune and eventually Marx. (I'd like to add I do not see the Enlightenment as wholesale detestable, but regardless of the good it also brought along with it much to disdain in hindsight). From then through the 1800s we began to see the slow dissolution of the social order, the extended family became the more capitalist-compatible nuclear family, personal liberty became desirable, rationalism was given priority as the way to ponder on the universe and the beginning of women's emancipation.

World War I was the catalyst of the effects that the Enlightenment brought upon the European empires, the competition that it fostered between them (escalated by a Enlightenment conception of ideas such as nationhood) finally became physical and their aristocratic governments used the sons of a generation as expendable pawns in a brutal war. It demonstrated the flaws of remnants of the archaic systems of old, it was now believed the people would probably be better making decisions. Modernism finally overcame tradition, this could be through the suffrage reforms of Lloyd George in Britain, the liberal revolution in Germany, the Bolshevik revolution in Russia. Plus it set the precedent upon which the world's second oldest and most well-developed ideologically liberal state the United States of America would become a dominant power. After this, we saw the first wave of the social changes of the 1900s. Women's suffrage became further spread, the hope of spreading literacy upon the whole population arose and we saw the prototypical stages of the sexual revolution. Of course, it was not as "happy" as this, the Great Depression came, Weimar fell to chaos and fascism arose. In the subsequent Great War we saw the last stand for opposition to the progressive liberalism of the Enlightenment in hopes of setting a new revolutionary path. This left the struggle between Jacobinism's son (Marxist socialism) vs Anglo-liberalism and Jacobinism's lovechild ("Free World" Capitalism). "Free World" Capitalism won, now we can finally contemplate on whether it was all worth it.

Judging from all this, the Protestant Reformation was the definite starting point (although you can go into the background of that and talk about the Renaissance's effects, proto-liberalism or proto-Protestantism) and the French Revolution really set the ball rolling. But I would say neither are really THE decisive point. While, I'd agree that the Bolshevik Revolution is important I wouldn't place that much importance on it. It is a symptom not a cause. I'd say it is one you didn't include, WW1. The Enlightenment's effects reaching their conclusion, ending in a cataclysm pushed on by archaic holdouts of a bygone age (aristocrats and monarchs). This all put ensured the final triumph of the progressive liberalism of the Enlightenment and began the post-Enlightenment age of neoliberalism, Bolshevism and progressivism.