The late Ottoman intellectual world was substantially occupied with debating Islam---disputing the definition of religion, its value and function, and historicity. The debates are studied in this paper through the lens of Mustafa Sabri Efendi (1869-1954), a sheikh al-Islam (head of the religious establishment), politician, and journalist. While Sabri Efendi engaged in controversial debates throughout his life, the primary focus of this study is his essays published in the journal Beyanul Hak. As the editor-in-chief of the journal Beyanul Hak (Expression of Truth), Mustafa Sabri wrote between 1908 and 1912 a series of essays, under the master title of Matters of discussion about the religion of Islam (din-i Islam’da hedefi munakasa olan mesail), on polygyny, the veil, divorce, inheritance and charity, insurance and gambling, effort and capital, depiction, and music. Also under scrutiny is his two books: Musa Carullah Bigiyef’e Reddiye (the Refutation of Musa Jarullah Bigiyev) in which he rejected the well-known Muslim Tatar reformist Jarullah’s ideas about divine mercy and the terminableness of the hell for nonbelievers; and Dini Muceddidler (Religious Innovators) where he offered a broader critique of the advocacy of religious reform. Although Mustafa Sabri is acknowledged as a foremost conservative and traditionalist figure, his writings in this study are examined primarily to find out his method of reasoning rather than to present his thoughts. That is to disclose on what grounds he disputed with others about religious matters and what kinds of argument he deployed to defend his views. His writings, this paper argues, can be analyzed as his manifesto to teach Muslims how to think about religion, Islam in particular. Mustafa Sabri underlined the importance of logic and the deployment of both rational and traditional proofs (the Prophetic sayings and Quranic verses) in explaining religious matters. However he also denied the dominant thought that religion has to be reasonable, easy and useful, and harmonious with human nature. The debates M. Sabri had with others reflect the tension embedded in the formation of religious orthodoxy and reform. They reveal how particular ideas and practices come to be authorized as the proper way of thinking and living Islam in the face of existing past and present rival formulations, which is formed out of the challenge to distinguish between the essence and minimum thresholds of the religious tradition and that which is not exempt from change.