Sandomierz Agreement

Facing counter-reformation, Jan Laski came with the idea of a united front, but it turned out to be a failure, when in the 1560s, the radical movement of the Polish Brethren emerged, dividing the Calvinists.

Due to efforts of Laski and Feliks Krzyzak, the agreement between the Calvinists and the Bohemian Brethren was signed in 1555 in Kozminek.

In Greater Poland, this task was taken over by Erazm Gliczner, while leaders of the local Bohemian Brethren were Jakub Ostrorog and Rafal Leszczynski.

The expansion of both Bohemian and Polish Brethren was stopped after 1577, and in the late 16th century, the Roman Catholic church managed to weaken the influence of the Calvinists in the Polish–Lithuanian Commonwealth.

The only Protestant creed that retained its position was Lutheran Church, which was very strong among German-speaking residents of Royal Prussia.