The Orthodox nobles' rights were nominally equal to those enjoyed by Polish and Lithuanian nobility, but there was a cultural pressure to convert to Catholicism, that was greatly eased in 1596 by the <answer>Union of Brest</answer>. See for example careers of Senator Adam Kisiel and Jerzy Franciszek Kulczycki.
What greatly eased the cultural pressure towards Catholicism. 
Union of Brest