In this study of the theology of Henri de Lubac, S.J., I examine de Lubac's confrontation with what he calls the "spiritual posterity" of Joachim of Fiore. I argue that this tradition is one of the principal phenomena against which de Lubac's theology is framed, showing that he interprets it primarily as a source for the atheism he faces in early twentieth century France and for the theological problems he identifies in the postconciliar Church. His judgment is that the theology of Joachim of Fiore and the spiritual movements he inspired lay the foundations for atheistic reinterpretations of Christian doctrine.I also develop a creative reconstruction of de Lubac's theological alternative to Joachimism—the part of his late work, La postérité spirituelle de Joachim de Flore, which he was unable to complete. I argue that de Lubac's confrontations with Joachimism allowed him to re-present his positions from earlier writings (on the Trinity, exegesis, the Church, and the eschaton) as always already providing an alternative to Joachim's teachings, as well as a response to his atheist interlocutors. The dissertation advances our knowledge of de Lubac by illuminating a hitherto poorly-explored area of his thought. It also adapts from this study new insights about the nature and purpose of fundamental theology, the theology of history, and the Church's engagement with the modern world.