The Czech brothers came to the Duchy of Prussia at the turn of 1548 and 1549. They settled mainly in Masuria (Nidzica, Działdowo, Dąbrówno and Olsztynek) and in Powiśle (Kwidzyn, Gardeja). However, they found themselves in quite difficult living and religious conditions. This is because their freedom of worship was largely restricted, subordinating them to the Evangelical Church. From around 1553, the return migration of members of the Unity began, which ended in the late 1970s. The vast majority of the brothers returned to the Czech Republic or Greater Poland, only a few were assimilated. The Czech brothers did not play a more serious role in Ducal Prussia, as in Poland, for example, but they were there for too short a time. It is timely to now approach the question of religious tolerance in Ducal Prussia. The case of the Czech brothers demonstrates that it was not actually there, on the contrary – one can speak of a reverse of tolerance, namely an attempt to confessionalize Augsburg-Lutheran Ducal Prussia. To put it simply, the term „confessionalization“ means a society and state consisting of one religion, the influence of religion not only on the spiritual life, but also on all manifestations of social, cultural and even political life. According to Janusz Małłek, we are dealing with consistent Lutheran confessionalization in Ducal Prussia until at least the 1620s. Only then was there a reduction – albeit a moderate one – in religious intolerance. These observations are only a preliminary attempt to answer the question posed in the title, which undoubtedly requires much more indepth research.