Fürstbistum Ermland
Martin Kromer and Stanislaus Hosius (Stanisław Hozjusz) were the two bishops most instrumental in causing Royal Prussia's diocese of Warmia to return to or remain Catholic during a time of major conversions to Protestantism, especially in the neighboring Duchy of Prussia which almost surrounded the Prince-Bishopric of Warmia, and had converted to Lutheranism as the first state to do so.