This article considers the effects of atheism, an intellectual and political movement denying the existence of God (the Supernatural) and casting doubt on the point of institutions connected with God in twentieth-century Bohemia and Moravia. The author distinguishes between atheist, agnostic, and “non-believer,” and, referring to contemporary sociological research into religiousness in Czech society, argues that it would be wrong to consider the mass turning away from traditional confessions to be evidence of its prevailing atheism or a consequence of forty years of Communist dictatorship. The article considers the topic in the broader historical context, and points to the anticlerical (essentially anti-Roman Catholic) tradition in modern Czech history, which is rooted in the National Revival and was intensified in connection with the anti-Habsburg struggle leading to the creation of the Czechoslovak Republic. The Communist regime, seeking, after it took power in February 1948, to suppress the Church and religion, thus found fertile ground. The beginning of atheism in the Czech milieu, as elsewhere in Europe, is linked to the development of the Freethinkers movement. Within this movement (the Czech section, Volná myšlenka, was founded in 1904), a positivist current predominated at first. From the early 1920s, however, its views increasingly clashed with the Marxist-influenced stream. That stream originated in Marx’s interpretation of religion as a false, alienated consciousness, serving the interests of reactionary social forces and an outdated “scientific view of the world”. Atheism in the Marxist conception was thus understood not only as a noetic perspective, but also as a set of principles forming part of Communist, or Socialist, ethics. The author argues that, after taking power, the Communist regime commenced its struggle against the Churches (particularly the Roman Catholic) with the help of propaganda that was political rather than atheist, owing both to tactical considerations (the considerable religiousness of the rural population) and to the implicit conviction of Communist functionaries that religion would die out together the people and institutions that represented it. In the 1950s, “scientific atheism” had not yet emerged from Marxist-Leninist doctrine as an independent discipline, and was therefore not a special subject of the school curriculum or scholarly debate. It emerged slowly, in about the 1960s, but by then, with the overall liberalization of society, relations between the Churches and State had improved, and space for religious and ideas had begun to appear. In the last part of the article, the author describes the institutionalization of “scientific atheism” as part of the strategy of “Normalization”, reflected for example in the founding the Institute of Scientific Atheism at the Czechoslovak Academy of Sciences, Brno, 1972. The mission of this institute was not merely the theoretical refutation of religion and the promotion of a “scientific view of the world” in research into the orientation of the population in this respect, but also the elaboration of assessments for publications with regard to their “ideological incorruptibility” and assessments of the activity of the clergy in deciding to revoke the requirement of State consent for those who wished to work as members of the clergy.