 Several encyclicals addressed the Eastern Catholic Churches . Orientalis Ecclesiae was issued in 1944 on the 15th centenary of the death of Cyril of Alexandria , a saint common to Eastern Christianity and Latin Churches . Pius XII asks for prayer for better understanding and unification of the Churches . Orientales omnes <unk> , issued in 1945 on the 350th anniversary of the reunion , is a call to continued unity of the Ruthenian Church , threatened in its very existence by the authorities of the Soviet Union . <unk> Rex was issued in 1951 on the 1500th anniversary of the Ecumenical Council of Chalcedon . It included a call to oriental communities adhering to Miaphysite theology to return to the Catholic Church . Orientales <unk> was issued in 1952 and addressed to the Eastern Churches , protesting the continued Stalinist persecution of the Church . Several Apostolic Letters were sent to the bishops in the East . On 13 May 1956 , Pope Pius addressed all bishops of the Eastern Rite . Mary , the mother of God , was the subject of encyclical letters to the people of Russia in Fulgens corona , as well as a papal letter to the people of Russia . 
