In Suprema Petri Apostoli Sede

In Suprema Petri Apostoli Sede (On the Supreme See of Peter the Apostle), also titled Litterae ad Orientales, i.e. Epistle to the Easterners, is a document – either considered as an apostolic letter or as an encyclical letter – sent by Pope Pius IX in 1848 to the bishops and clergy of the Eastern Orthodox Churches urging them to enter in communion with the Roman Catholic Church.[1][2][3]

The document was answered the same year by several leading members of the Eastern Orthodox Church, in the Encyclical of the Eastern Patriarchs, which restated the theological controversies between the two Churches, most notably the dispute over the addition of the Filioque to the Nicene Creed and papal supremacy.