Most historians have assumed a fundamental antagonism between Marxism and theism. In practice, the relationship between the two world-views has been far more complex than simple hostility – a complexity admirably illustrated by the experience of the Marxist Parti Ouvrier Français (POF) between 1882 and 1905. While the Marxists of the POF developed a vicious socialist anti-clericalism that made its own original contribution to France's long tradition of anti-religious polemic, they none the less experimented with a rudimentary Christian socialism designed to attract the proletarian faithful, and also developed an agnostic programme of religious indifference which sought to insert the circuit-breaker of class conflict into the highly charged link between militant secularism and Catholic clericalism. This article examines the intricate and, in the end, incoherent, pattern of engagement between Marxist socialism and French religion during the fin de siècle, and suggests that this incoherence contributed to the eventual frustration of the Parti Ouvrier's revolutionary purpose.