The article is devoted to the study of the views of the Metropolitan of Kiev Sylvester Kosov on the essence, structure and function of the ideal state. An attempt was made to determine how these views influenced the formation of the self-identification of the Ukrainian ethnos during its design in the early modern nation. It was found out that the state concept of S. Kosov was formed on the basis of scientific achievements of the scientific community of P. Mogila and was influenced by the Polish political ideology of the end of XVI - beginning of the XVII century. It is determined, that the nearest analogies particular concept had in the works of A. Kallonofyskiy and C. Copistensky. It is argued that one of the achievements of S. Kosov is the formulation of the ideologue of Kiev as the Second Jerusalem, which justified the political right to the sovereignty of the Ukrainian state. The political nation of that state considered to be the "people of Rus" as an ethnic community sharing the values of the baptism of Rus prince Volodymyr. The ruling elite of the ideal Ukrainian state considered the Orthodox gentry to be the metropolitan, and some of the functions of management recognized the Cossacks. The ideal ruler in the theory of S. Kosov was the "king-philosopher", endowed with wisdom and moral virtues. It was found out that S. Kosov one of the first introduced in the structure of self-identification of the Ukrainian nation the category of "antiquities" as the historical duration of the people, state and political elite. S. Kosov's views had a serious impact on the formation of the self-awareness of the early modern Ukrainian nation and were partially implemented in the political activities of B. Khmelnytsky. However, there was a significant divergence between the ideology of the Orthodox clergy and the Cossacks, which became heightened later in the times of the Ruins.