I can answer your question in regards to (German language) intellectual history. The first famous skeptic of religion was Ludwig Feuerbach (1804 –1872). His main work, The Essence Of Christianity came out in 1841 and presented an anthropomorphized, purely cultural theory of Christianity. He strongly influenced Marx and Nietzsche, the other famous religious skeptics of the 19th century. Not that being an atheist was necessarily easy back then but the fact that you could publically hold such a position marks a tremendous shift in the cultural perception of religion. Even just a hundred years prior, someone publically holding a position like Feuerbach was utterly unthinkable. A few things contributed to this change. The first one was the spread of Spinozism via the "Pantheism controversy", one of the most important intellectual events of the German enlightment (the controversy errupted because Lessing, a key enlightment figure, declared himself a commited Spinozist). While Spinoza doesn't hold a literal atheism like Feuerbach, his God, immanent and idiosyncratic, is significantly opposed to the traditional personal God of Christianity. Spinoza was a key influence to the German Idealists (Kant, Hegel, Fichte, Schelling), especially Hegel, who in turn heavily influenced Feuerbach and Marx.

The German Idealists were all very commited Christians but their conceptions of Christianity are without a doubt idiosyncratic and opposed to dogmatic Catholic teachings. For example, Kant, an incredibly pious Christian, argued for a Christianity stripped of any sort of "religion of revelation" and argued for a private, modest "religion within the limits of bare reason". Such a radically protestant and enlightment position was only made possible by the final downfall of absolute Catholic religious authority following the reformation and the chaos of the Thirty Years War. This also allowed for a spread of religious pluralism in the form of amateur theologicians like Jakob Böhme, who developed an incredibly idiosyncratic, mystic conception of Christianity that was hugely influential to the German enlightment and romanticism. Another key factor is the early modern obsession with antiquity and Greek / Roman culture, which was frequently muddled together with Christian concepts as well as the emergence of "Orientalism", the study and translation of religious texts from the far east, which was important to figurs like Schopenhauer and Schelling. And of course, the progress of the natural sciences had significantly transformed the pre-modern spiritual understanding of a world towards mechanism and materialism (see: "materialist controversy"). Ultimately, all of these points slowly lead to a decline of religious authority and a spread of theistic pluralism that stripped down Christianity to the point where there wasn't really any universally agreed upon, unifying dogma left among the educated classes. Downright proclaiming yourself to be an atheist was, without a doubt, still a radical step but dogmatic Christianity had been slowly undermined enough to where it became possible.