"Pope & Patriarchs: The 1848 Letters of Pope Pius IX and the Orthodox Patriarchs"

This amazing compilation comes from the website OrthoCath blog. 

Shortly after his ascension to the papal throne in 1846, Pope Pius IX wrote the Apostolic Letter In suprema Petri apostoli sede,  “On the Supreme Throne of Peter the Apostle” (6 January 1848). While it was primarily intended for Eastern Catholics of the various Oriental Rites it also addressed Orthodox Christians, calling them back to unity with Rome.

Fr. Aidan Nichols characterizes Pius IX’s letter as

“the first ‘unionist’ encyclical of the modern papacy” and as the first part of  “a papal concern for the Christian East of a depth and urgency not seen since Florence.” “Thousands upon thousands” of this letter were distributed in a Greek translation directly to the Orthodox faithful. (Rome and the Eastern Churches, p. 352).

Despite its historical importance as one of the first texts of the modern era relative to Catholic—Orthodox relations, English translations of Pius IX’s letter have been few and, until now, have not been available online.

A translation from the French version, published in the journal Irenikon in 1929, has been prepared by Michaël de Verteuil. The French text can be found here at the Internet Archive and was supplied by the Monastery of Chevetogne, publishers of Irenikon.

The reply to Pius IX’s letter, issued by the Orthodox Patriarchs later that year, is better known and is online here and used with permission.

The Orthodox Patriarchs’ reply was signed by Anthimos VI, the Ecumenical Patriarch of Constantinople, along with Hierotheus, Patriarch of Alexandria, Methodios, Patriarch of Antioch and Cyril, Patriarch of Jerusalem, along with the Holy Synods of each of the Patriarchates.

Read the entirety of the whole thing at Popes and Patriarchs

This is true statement of the Orthodox stance against papist authority.