This dissertation examines the application of Islamic theology (kalām) and philosophy to the elaboration of Almohadism, a new form of Islam founded in North Africa by Muḥammad Ibn Tūmart (ca. 1080-1130). There has been a consistent interest in Ibn Tūmart’s role as the progenitor of the Almohad Caliphate (1121-1269) and his claim to be the infallible imām and awaited mahdī, a messianic figure within Islam. However, most treatments of Ibn Tūmart’s religious thought have relied on historical accounts rather than the extant body of writings attributed to him, The Book of Ibn Tūmart (BIT). Using manuscript evidence and an array of Arabic and Latin sources, I confront questions about the authenticity of this compilation of Ibn Tūmart’s writings. I conclude that Ibn Tūmart authored all texts in BIT (except the few explicitly attributed to other hands) and offer a new arrangement of the text that reconciles discrepancies between the two extant manuscripts. I propose a new reading of the text based on the structure imparted to the compilation by its compiler. BIT begins with an analysis of the epistemological principles of the Islamic sciences—especially jurisprudence and theology. Ibn Tūmart’s epistemology is remarkable for its deep commitment to rationalism in speculative theology and the interpretation of the Qur’ān and ḥadīth. On these principles, it proceeds to an exposition of divine unity (tawḥīd), a theory of religious leadership (imāma), and a program of missionary activity and militant jihād under the belief that Ibn Tūmart possesses exclusive authority to adjudicate among and teach all Muslims. Throughout this dissertation, I aim to contextualize Ibn Tūmart’s thought within contemporary intellectual developments, particularly the philosophy of Ibn Sīnā (d. 1037) and his Ash‘arī reception by al-Juwaynī, al-Ghazālī, and others. I argue that, although Ibn Tūmart emerged from a Sunnī background, he adopted minoritarian views within Sunnism and applied them in a radical way. These views include the idea that one’s salvation is predicated on knowledge of (rather than mere belief in) God and the rejection of the Sunnī principle of infallibilism in legal reasoning (ijtihād). These positions, along with Ibn Tūmart’s claim to be the awaited mahdī, put the Almohad movement into conflict with North African Sunnism. The Almohads’ struggle to impose a rationalist vision of Islam on twelfth-century Morocco culminated in several bloody purges of those who opposed them in favor of a traditionalist form of Islam. Within Ibn Tūmart’s writings, we see a push toward popular enlightenment, an outgrowth of his belief that salvation requires knowledge. I conclude the dissertation with a discussion of Almohadism’s decline and its influence on intellectual life in Morocco and al-Andalus. Almohad Islam, likely due to its radicalism, failed to maintain its viability as a new form of Islam separate from Sunnism because it failed to win enough converts among local scholars to effect a lasting demographic shift. However, Ibn Tūmart’s mahdism and emphasis on popular enlightenment provoked skepticism in the following centuries toward the role of religious rationalism within Islamic politics.