Manichaeism’s most enduring impact on medieval religious thought lay less in the survival of its own communities and more in the way it became the archetype of “false teaching.” Christian authorities, both in the Latin West and the Byzantine East, treated it as the classic example of radical dualism: two opposed principles, spirit and matter, light and darkness. The very word “Manichaean” came to function as a kind of shorthand for any position that seemed to deny the goodness of creation, reject the Old Testament God, or posit a rival power alongside the one God. In this way, Manichaeism offered a conceptual template by which orthodoxy defined itself over against a clearly marked “other.”

This template shaped how later movements were perceived and condemned. Groups such as the Paulicians, the Bogomils, and especially the Cathars were frequently described as “neo‑Manichaean,” whether or not the historical links were strong. The Church’s polemical literature, inquisitorial procedures, and legal measures drew on earlier experience with Manichaean communities, whose hierarchical structures, secret teachings, and dualistic cosmology had already been mapped and refuted. As a result, accusations of “Manichaeism” became a powerful tool in heresy debates, allowing diverse forms of dissent to be grouped under a single, ominous label.

At the same time, Manichaeism exerted a deep, if indirect, influence on Christian theology itself. Augustine of Hippo, who had once been associated with the Manichaeans, developed his mature thought in sustained opposition to their dualism and determinism. His insistence that evil is not a positive substance but a privation of good, and his defense of human responsibility against cosmic fatalism, became foundational for later medieval discussions of divine omnipotence, free will, and the problem of evil. Medieval theologians repeatedly drew on this anti‑Manichaean framework when confronting both philosophical questions and concrete heresy trials.

Beyond the Latin West, Manichaeism also served as a foil in Eastern Christian and Islamic contexts. Byzantine theologians and imperial authorities used it as a benchmark for doctrinal deviance, while Muslim thinkers composed refutations of its dualism in order to clarify and defend the unity of God and the created order. Across these different settings, Manichaeism’s lasting legacy was to sharpen the contours of orthodoxy: by standing as the paradigmatic dualist faith, it compelled religious thinkers to articulate more carefully what it means to affirm one good Creator, a meaningful human freedom, and a world that, despite its shadows, is not abandoned to an equal and opposite darkness.