Because:
"The new doctrine, that “the Holy Ghost proceedeth from the Father and the Son,” is contrary to the memorable declaration of our LORD, emphatically made respecting it: which proceedeth from the Father (John xv. 26), and contrary to the universal Confession of the Catholic Church as witnessed by the seven Ecumenical Councils, uttering “which proceedeth from the Father.” (Symbol of Faith).
i. This novel opinion destroys the oneness from the One cause, and the diverse origin of the Persons of the Blessed Trinity, both of which are witnessed to in the Gospel.
ii. Even into the divine Hypostases or Persons of the Trinity, of equal power and equally to be adored, it introduces diverse and unequal relations, with a confusion or commingling of them.
…
iv. It censures the holy Fathers of the first Ecumenical Synod of Nicea and of the second Ecumenical Synod at Constantinople, as imperfectly expressing what relates to the Son and Holy Ghost, as if they had been silent respecting the peculiar property of each Person of the Godhead, when it was necessary that all their divine properties should be expressed against the Arians and Macedonians.
v. It reproaches the Fathers of the third, fourth, fifth, sixth, and seventh Ecumenical Councils, which had published over the world a divine Creed, perfect and complete, and interdicted under dread anathemas and penalties not removed, all addition, or diminution, or alteration, or variation in the smallest particular of it, by themselves or any whomsoever. Yet was this quickly to be corrected and augmented, and consequently the whole theological doctrine of the Catholic Fathers was to be subjected to change, as if, forsooth, a new property even in regard to the three Persons of the Blessed Trinity had been revealed.
vi. It clandestinely found an entrance at first in the Churches of the West, “a wolf in sheep’s clothing,” that is, under the signification not of procession, according to the Greek meaning in the Gospel and the Creed, but under the signification of mission, as Pope Martin explained it to the Confessor Maximus, and as Anastasius the Librarian explained it to John VIII.
vii. It exhibits incomparable boldness, acting without authority, and forcibly puts a false stamp upon the Creed, which is the common inheritance of Christianity.
viii. It has introduced huge disturbances into the peaceful Church of God, and divided the nations.
ix. It was publicly proscribed, at its first promulgation, by two ever-to-be-remembered Popes, Leo III and John VIII, the latter of whom, in his epistle to the blessed Photius, classes with Judas those who first brought the interpolation into the Creed.
x. It has been condemned by many Holy Councils of the four Patriarchs of the East.
xi. It was subjected to anathema, as a novelty and augmentation of the Creed, by the eighth Ecumenical Council, congregated at Constantinople for the pacification of the Eastern and Western Churches.
xii. As soon as it was introduced into the Churches of the West it brought forth disgraceful fruits, bringing with it, little by little, other novelties, for the most part contrary to the express commands of our Savior in the Gospel—commands which till its entrance into the Churches were closely observed…
xiii. It drove the theologians of the West, as its defenders, since they had no ground either in Scripture or the Fathers to countenance heretical teachings, not only into misrepresentations of the Scriptures, such as are seen in none of the Fathers of the Holy Catholic Church, but also into adulterations of the sacred and pure writings of the Fathers alike of the East and West.
xiv. It seemed strange, unheard of, and blasphemous, even to those reputed Christian communions, which, before its origin, had been for other just causes for ages cut off from the Catholic fold.
xv. It has not yet been even plausibly defended out of the Scriptures, or with the least reason out of the Fathers, from the accusations brought against it, notwithstanding all the zeal and efforts of its supporters. The doctrine bears all the marks of error arising out of its nature and peculiarities. 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 St. 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 St. 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."
orthodoxinfo.com/ecumenism/encyc_1848.aspx
