The article specifies the role of rationalism in the work of a distinguished Baroque author, Łukasz Opaliński. The source material for the study consists of: "Rozmowa Plebana z Ziemianinem" (1641), "Polonia defensa contra Ioan[nem] Barclaium" (1648), "Coś nowego" (1652), "Pauli Naeoceli de officiis libri tres" (1659), "Poeta nowy" (1661–1662). Łukasz Opaliński stated that reason is the most important cognitive faculty of the human being. Reason makes it possible to understand the rules of natural law, which is the source of ethical action among humans, and brings on understanding of the eschatological aim of human life, that is salvation. The possibilities of rational explanation do not, however, encompass the mysteries of religious faith o...