This paper examines a relationship between heresy and utopianism forged in nineteenth- and early twentieth-century socialist histories to reveal a significant and pervasive fault-line in the ideological construction of anarchism. I look at Marxist narratives which trace the lineages of socialism to medieval religious dissent and show how the sympathetic assessment of European heretical movements was moulded by a critique of utopianism, understood as the rejection of materialist ‘science’. I argue that strands of this narrative have been woven into anarchism by looking at three accounts: E.V. Zenker’s Anarchism (1897), James Joll’s The Anarchists (1964/1979) and Saul Newman’s From Bakunin to Lacan (2001). Their dominant theme is that anarchism promises the transformation of corrupted nature, typically achieved though ecstatic violence, cataclysmic revolution and future perfection. I describe this Millenarian anarchism as a ‘straw man’ but rather than jettison ‘heresy’ as an investigative tool, I refer to a conception of heresy as choosing to present an alternative account. Using Martin Buber’s analysis of utopianism in Paths in Utopia (1949) and Michael Bakunin’s critique of political theology, I pair utopianism with the rejection of perfection and heresy with faith. This reframing of heresy corrects a deep-rooted, long-standing distortion of anarchist ideas.