This paper examines Fakhr al-Dīn al-Rāzī’s understanding of metaphysical certainty in terms of his theory of ta’wīl (interpretation) while showing his optimism in attaining metaphysical certainty. Rāzī, also known as the leader of the skeptics (shaykh al-mushakkikīn) in the Shiʻi sources, while thoroughly criticizing the philosophical and kalam traditions before him, remains a controversial figure among scholars. His critical thinking confounded subsequent thinkers, and thus, various ways of reading about Rāzī have emerged. Some have evaluated Rāzī as a metaphysical agnostic who believed that the intellect cannot attain certainty in theological knowledge. This study positions Rāzī’s account of metaphysical certainty in relation to his theory of ta’wīl. The first part of the article focuses on the history of the relationship between metaphysical certainty and ta’wīl the debates over the relationship between intellect and transmission in theological knowledge and offers the historical context in which Rāzī developed his idiosyncratic approach. The second part identifies Rāzī’s principles of reason in metaphysical knowledge through the interpretation of the concept of istiwā’. This article does not aim to fully investigate Rāzī’s understanding of ta’wīl. However, it analyzes how intellectual truths, one of the main components of the theory of ta’wīl, become metaphysical certainties. The Muʻtazilī mutakallimūn made metaphysical certainties, which are transformed from intellectual truths, a yardstick of understanding and interpreting religion. On the other hand, what some might call their obsession with reliance upon metaphysical certainties became an intolerant attitude towards different interpretations of religion, grew into an oppressive ideology with political power, and ultimately fueled a critical resistance by non-Muʻtazila scholars against rationality (or even rationalism itself). As a natural consequence, the rational development of other doctrines was slowed down by the reaction against Muʻtazilī influence. The first part of the article, while discussing Kalam schools, especially the Ashʻarī school of theology, in terms of metaphysical certainty and the interpretation of revelation, charts the crystallization of the Ashʻarī account of the relationship between interpretation (ta’wīl) and intellectual truths, a historical process inversely correlated with the presence of the Muʻtazila. However, the crystallization process, which was somewhat ambivalent until Rāzī, reaches its ultimate form with Rāzī. The first of the main principles of Rāzī’s theory of ta’wīl is that the intellect is the foundation of revelation (al-ʿaql aṣl al-naql). The intellect becomes the decisive factor not only in terms of authentication and understanding of revelation but also in terms of its interpretation (ta’wīl). Focusing on his Tafsīr, one of his last treatises and which was left incomplete, this article argues against the claim that toward the end of his life, he was inclined to metaphysical agnosticism, falling into an epistemic pessimism with respect to attaining metaphysical certainty. Rāzī takes a firm stance on the probability of transmission in works written throughout his life. Rāzī’s firm stance on the probability of transmitted sources necessarily leads to the principle that reason is the foundation of transmission. Especially with his account of ta’wīl, he offers a rational theology in which he maintains his optimism on metaphysical certainty.