ENThe study is dealing with popular Calvinistic catechism Coś ty jest?, which was a part of traditional hymn books from the middle of the sixteenth century up to the 1640-th., and formed a basis for religious instruction of the faithful. Coś ty jest? comes into being under the influence of different confessional traditions: the sources of principle importance are the catechetical works of the Bohemian Brethren and Lutheran theologian Johannes Brenz. Evolution of Coś ty jest? reflects the progress of dogmatic profile of the Calvinism in the Polish-Lithuanian Commonwealth, which was changing in polemics with the Lutherans and the Bohemian Brethren, but also with the follows of the anti-Trinitarianism.