In tracing a relatively unknown but important feature of the work of Friedrich Engels, this article offers a critical commentary on his lifelong engagement with the New Testament book of Revelation. Beginning with material from his late teens, when he was undergoing the long, slow process of giving up his Calvinist faith, Engels used the text for humor and satire, for polemics, and as a way to express his own exuberance. As the years unfolded, he would come to appreciate this biblical book in a very different fashion, namely, as a historical document that offered a window into earliest Christianity. Through three essays, one on Revelation, another on Bruno Bauer (from whom Engels drew increasingly as he grew older), and a third on early Christianity, Engels developed the influential argument that Christianity had revolutionary origins. In closing, I ask three questions: What is the abiding relevance of Engels‘s work? Where does it fall short? And what tensions does he open up in his thought by allowing ideas and beliefs to influence history?