The article discusses little-known facts from the lives of two great representatives of the Silver Age of Russian philosophy – Nikolai Berdyaev and Sergei Bulgakov – referring to the period when both were ardent Marxists. It discusses the beginning of the academic career of both thinkers, the turn of the 19th and 20th centuries. In the archives of Karl and Luise Kautsky in Amsterdam (International Institute of Social History) there are two Berdyaev’s letters to Kautsky regarding polemics about Marx and Marxism, which unfolded between them after Kautsky’s decision to publish in the pages of Die Neue Zeit an article by Berdyaev “F.A. Lange and Critical Philosophy in Its Relation to Socialism” (1900). This correspondence has probably become the catalyst for Berdyaev’s transition from ‛orthodox’ to ‛critical’ Marxism. On the other hand, Bulgakov’s letters to Kautsky (and those of his wife, Helena Tokmakova, to Luisa Kautsky) refer to the time of a research internship of Bulgakov in Berlin in the years 1898–1900. He then met Kautsky and Bernstein families, and engulfed himself in theoretical problems of Marxism. The text of the speech is accompanied by a translation into Polish and provided with comments on two Berdyaev’s letters to Kautsky (February and May 1900).