This lesson introduces Uṣūl al-Dīn (أصول الدين) as the foundational principles upon which the Islamic religion is built, with al-Tawḥīd (التوحيد) at its core. Linguistically, al-aṣl (الأصل) refers to that upon which something else is built, and technically it is used by jurists and theologians to mean evidence, a foundational principle, or an established presumption. In Islam, Uṣūl al-Dīn centers on belief in Allah, His oneness, and the realities of the beginning (al-mabdaʾ) and return (al-maʿād). For this reason, the science that studies these beliefs came to be known by several names throughout history, such as al-Fiqh al-Akbar (الفقه الأكبر), al-Īmān (الإيمان), al-Sunnah (السنة), al-Tawḥīd, al-ʿAqīdah (العقيدة), and Uṣūl al-Dīn, all referring—broadly—to the discipline concerned with establishing sound belief.

The lesson explains that ʿIlm al-Kalām (علم الكلام) aims to affirm religious truths through decisive proofs and to repel doubts and misconceptions. Classical definitions describe it as knowledge by which one is able to establish correct beliefs through evidence, distinguishing it from blind imitation (taqlīd). Its subject matter is primarily the “states of the Creator” (aḥwāl al-Ṣāniʿ), including divine attributes, actions, prophethood, revelation, and the Hereafter, while also addressing created things insofar as they point to the existence, oneness, and perfection of Allah. This science is regarded as the noblest of sciences because correct worship depends on correct belief, and without īmān, no amount of outward action is ultimately beneficial.

Finally, the lesson traces the historical development of Uṣūl al-Dīn from the prophetic era onward. The Makkan Qurʾān focused on establishing core beliefs through reflection on creation, rational argument, stories of the prophets, and affirmation of resurrection and accountability. The Sunnah further embodied and clarified these beliefs through the Prophet’s ﷺ character, teachings, and responses to doubts. The Companions (Ṣaḥābah) received these doctrines with submission, relying primarily on revelation while using reason to understand—not override—the texts. Early deviations, such as the roots of Khārijite extremism or false claims to prophethood, were firmly addressed, and later disagreements among the Companions remained within the bounds of legitimate ijtihād. Altogether, this lesson shows how Uṣūl al-Dīn was established, preserved, and defended as the unchanging foundation of the Islamic faith.