The focus of the article is an analysis and interpretation of the dramatic transformation of the Unity of the Brethren at the end of the 15th century. In this period, the Unity of the Brethren primarily reconsidered its rejection of secular power, the urban way of life, and most crafts and trade. These changes are referred to in previous research as the transformation of the sect into a church. Nevertheless, this transformation has not yet been systematically studied using adequate theoretical methods that would reflect the more general mechanisms of development operating in Christian religious movements. The author therefore returns to the sociological roots of this debate. Inspired by the neo-Weberian approach of David d'Avray, the author uses theories concerning the transformation of sects into churches as ideal types in the comparison of two key thinkers – Petr Chelčický and Luke of Prague. Petr Chelčický inspired the establishment of the Unity of the Brethren and his teachings rejected secular power and the urban way of life. In contrast, Luke of Prague legitimized the abovementioned changes in his teaching. The analysis reveals that the tension between the Unity of the Brethren and mainstream society was not diminished, as it was closely linked to the identity of the Unity of the Brethren. On this level, the Unity of the Brethren remains more of a sect. However, the new concept of salvation by Luke of Prague, based on God's grace, enabled and legitimized life in cities, participation in secular power, and the transformation of social practice, without disturbing the sense of exclusivity of the Unity of the Brethren.