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 Union of Brest. See for example careers of Senator Adam Kisiel and Jerzy Franciszek Kulczycki.
Try to answer this question if possible (otherwise reply "unanswerable"): What other rights were equal to orthodox nobles?
Polish and Lithuanian nobility