The present paper investigates the functions of reason in the realm of religion in the kalami thoughts of Qadi Abd al-Jabbar Mu‘tazili and Qadi Abubakr Baqillani following a descriptive-analytic method. In doing so, it compares and examines the rational approaches of both thinkers to the interpretation of the Qur’an and applications of reason in inferring religious principles. The results of this study indicate that what distinguishes these two great figures from each other more than anything else is their approach to reason and the quality of its relationship with revelation. Qadi Abd al-Jabbar believes in the priority of reason and rational arguments and always resorts to reason as a tool for gaining knowledge in his kalami perception of religion. Sometimes, in cases where rational judgment is in contrast to the exoteric meaning of Qur’anic verses and traditions, he even gives the priority to reason with no reservation and firmly interprets or negates the validity of propositions which stand against reason. However, preferring tradition to the intellect and granting priority to the descended texts, including the Qur’an, traditions, and those on the acts of the Prophet’s companions are the most important epistemological principles of Baqillani. However, his philosophical system, in fact, marked the beginning of Ash‘arite kalam’s treatment of rational premises. The Ash‘arite considered rational principles to depend on beliefs and, thus, believed that it was first necessary to have faith in their content.