The Encyclical of the Eastern Patriarchs is a letter issued in May 1848 by the four eastern patriarchs of the Eastern Orthodox Church, who met at Council in Constantinople. It was addressed to all Eastern Orthodox Christians, as a response against Pope Pius IX's Epistle to the Easterners, issued in January (1848).
The encyclical was solemnly addressed to "All the Bishops Everywhere, Beloved in the Holy Ghost, Our Venerable, Most Dear Brethren; and to their Most Pious Clergy; and to All the Genuine Orthodox Sons of the One, Holy, Catholic, and Apostolic Church." The encyclical explicitly denounces the Filioque clause added by Rome to the Nicene Creed as a heresy, censures the papacy for missionizing among Eastern Orthodox Christians, and repudiates Ultramontanism (papal supremacy). It also describes the Roman Catholic Church as being in apostasy, heresy, and schism.
In the course of all this, it notably makes reference to the Eighth Ecumenical Council (879-880).