Research on Marxism as a religious movement has primarily focused on its theological implications. Building on this work, this article instead examines the practical aspects of revolutionary Marxism as a religious experience during the Latin American Cold War, and compares it to two other non-Christian religious traditions, Judaism and Umbanda. Drawing on secret police records, memoirs and oral history interviews, this article explores the influence of Judaism, Marxism and Umbanda on the anti-dictatorship activism of Alfredo Syrkis. Through an analysis of Syrkis’s life history, it assesses his conversion from liberal anticommunism to revolutionary Marxism, his participation in Marxist proselytising as a high school activist and his political activity in the clandestine Marxist organisation Revolutionary Popular Vanguard (VPR), highlighting group dynamics that were comparable to millenarian movements. It also considers the importance of other religious traditions in Syrkis’s life, including Judaism, the religion of his parents that equipped him with valuable social ties, and Umbanda, a syncretic Afro-Brazilian religion that Syrkis turned to during times of extreme anxiety in the armed struggle. This article argues that the religious traditions of Judaism, revolutionary Marxism and Umbanda influenced Syrkis’s political activism in both complementary and competing ways. While none of these traditions were able to command Syrkis’s undivided loyalty, collectively they informed the terms of his engagement with and disengagement from the Brazilian armed struggle against military rule. By analysing Syrkis’s life history through the lens of religion, this article broadens the study of cultures of militancy during Latin America’s Cold War.