This article represents the first systematic grounding of comparative theology in the sources of Shiite Islam. While contemporary comparative theology has taken form mainly within the framework of Christian teachings and by Christian comparative theologians, particularly Francis X. Clooney, Catherine Cornille, and Klaus von Stosch, no parallel effort has been made to ground this ever-growing discipline in the foundational sources of Shiite Islam. This article fills that scholarly gap by drawing on the primary sources of Shiite Islam, namely the Qur’an, the hadiths of the infallibles, and reason. These sources are explored for their identifying elements, teachings, and principles for learning across religious borders within the framework of Shiite Islamic orthodoxy and orthopraxy. The study demonstrates that the Qur’an reveals truths in the sacred texts of pre-Islamic religions. Furthermore, hadiths of the infallibles encourage us to seek truth, knowledge, and wisdom in any source. In addition, rational principles such as tarakum al-zunun, intertextuality, and tu’raf al-ashya’ bi-addadiha provide the necessary methodological background for encountering sources of other religious traditions. These extra sources are equivalent to the loci alieni in Catholic Christianity. The article argues that fundamental Shiite Islamic sources provide a legitimate framework for grounding a confessional comparative theology and open the horizon for rereading and enriching Shiite Islamic theological thought through constructive engagement and dialogue with other religious traditions. The article paves the way for Shiite Muslim theologians to participate more fully in the ever-expanding interdenominational project of comparative theology.