The author focuses on a previously unknown text by Musa Bigiev (died 1949), one of the greatest Russian religious figures of the first half of the 20th century, entitled “The ABC of the Great Truths of Islam”. The text was found in the materials of the Eastern Department of the GPU-OGPU and is devoted to the theological substantiation of the consistency of the ideas of communism and Islam. For the first time in Russian historiography, the article gives a general characterization of the source, its main provisions, and examines the circumstances of its appearance. The text of the “ABC” consists of an introduction and five chapters, divided into paragraphs, which are devoted to separate issues of combinability of the basic attitudes of communism and Islam. The theses are supported by theological argumentation, references to the Koran and hadiths, and statements of authoritative mujtahids. Bigiev concludes that there is a fundamental coincidence between communist attitudes and the values of Islam. The essay also contains detailed statements in favor of the cooperation of Russian Muslims and co-religionists of various Eastern countries around the world with the Bolsheviks, and a call for the creation of an “Eastern Union” to counteract the “imperialist West”. In conclusion, the author draws an inference on historical conditionality of the appearance of such a work by Bigiev in this historical period. “The ABC of the Great Truths of Islam” allows us to place Bigiev among the world theorists of Islamic socialist thought and to call him a ‘Muslim’ socio-political figure of the anti-colonial and anti-Western movement of the first half of the 20th century.