In 1848, Pope Pius IX wrote an Epistle to the Easterns, calling for the Orthodox Church to reunify with Rome. The same year, the Orthodox Church responded with the Encyclical of the Eastern Patriarchs.
This letter was signed by the Patriarch of Constantinople, the Patriarch of Alexandria, the Patriarch of Jerusalem, the Patriarch of Antioch–the heads of all four ancient Patriarchates–and twenty-nine other bishops besides. In this letter, we read:
All erroneous doctrine touching the Catholic truth of the Blessed Trinity, and the origin of the divine Persons, and the subsistence of the Holy Ghost, is and is called heresy, and they who so hold are deemed heretics, according to the sentence of Saint Damasus, Pope of Rome, who says: “If any one rightly holds concerning the Father and the Son, yet holds not rightly of the Holy Ghost, he is an heretic” (Cath. Conf. of Faith which Pope Damasus sent to Paulinus, Bishop of Thessalonica). Wherefore the One, Holy, Catholic, and Apostolic Church, following in the steps of the holy Fathers, both Eastern and Western, proclaimed of old to our progenitors and again teaches today synodically, that the said novel doctrine of the Holy Ghost proceeding from the Father and the Son is essentially heresy, and its maintainers, whoever they be, are heretics, according to the sentence of Pope Saint Damasus, and that the congregations of such are also heretical, and that all spiritual communion in worship of the orthodox sons of the Catholic Church with such is unlawful. Such is the force of the seventh Canon of the third Ecumenical Council.
The Orthodox Church has made it clear.
The filioque is neither a sideline issue nor a debatable doctrine.
It officially has been recognized as a heresy.