The article discusses the works on Fiqh of the famous Tatar theologian and enlightener of the 19th century Shihab ad-Din al-Marjani (1818-1889). In his works, Shihab ad-din al-Mardjani, in particular, discusses almost all aspects of Islamic law. In our article, we showed two examples: how Mardjani discusses the names of Muslims, and the authenticity of the tafsirs (interpretation) of the Qur’an. These examples show the author’s deep knowledge of the whole spectrum of Shariah problems as a whole, as well as each issue individually, with a deep study of sources on the Muslim law, the author’s judgments regarding the reliability of a certain source, and the desire to comply with Islamic law carefully, as it is spelled out in sacred texts of Islam. Marjani’s thoughts are relevant in our time, since this theologian, in interpreting the provisions of the Sharia, primarily relies on the textual sources such as the Qur'an and the Sunnah, thus being a supporter of ‘scholastic Islam’. The purpose of this work is to identify the prominent role of Shihab ad-Din al-Marjani in the development of the Fiqh science among Kazan Tatars and Muslims of Russia and of the world as a whole. To do this, it is necessary to review his works on Fiqh and, using concrete examples, to consider the depth of the elaboration of each issue under consideration in terms of attracting reliable sources according to Sharia, as well as the author’s ability to discourse, analyze and draw right conclusions.Keywords: Marjani, Qur'an, Sunna, names of God, tafsir, Muslim religious thought.