In Spain in 589 AD, a local synod, in violation of the Canons of the Church, added the word filioque (lit.: and the son) to the Nicene Creed asserting that the Holy Spirit proceeds from the Father and the Son. In the 9th century, the Frankish King Charlemagne, seeking to divide the Western half of the Roman Empire from the Eastern half in order to establish his own empire, attacked the Church in the East. He began to lodge accusations of heresy against Christians in the East. In order to further create divisions between Romans of the East and Romans of the West, he called those in the East “Greeks” which means “barbarians” and referred only to those in the West as Romans. (The so-called Greeks were Roman citizens of the Roman Empire, which Empire had as its capital the Eastern city of Constantinople. To this day the Greeks in Turkey are called the Rum or Rumi). He accused the Greeks of false doctrine for using the Creed as originally given, i.e., without the filioque. At the same time, he began to pressure the Apostolic See in Rome. As the eminent Theologian and Church Historian, John Romanides has written:
An unsuccessful attempt was made on the life of (the Roman) Pope Leo III (795-816), the successor of Hadrian. Pope Leo was then accused of immoral conduct. Charlemagne took a personal and active interest in the investigations, which caused Leo to be brought to him in Paderborn. Leo was sent back to Rome, followed by Charlemagne, who continued the investigations. The Frankish king required finally that Leo swear his innocence on the Bible, which he did on December 23, (800). Two days later Leo crowned Charlemagne “Emperor of the Romans.” Charlemagne had arranged to get the title “Emperor” in exchange for Leo’s exoneration. Charlemagne caused the filioque (the new line in the Creed that said that the Holy Spirit, "proceeds from the Father and the Son," instead of the original which read, "proceeds from the Father,”) to be added to the Frankish Creed, without consulting the pope. When the controversy over this addition broke out in Jerusalem, Charlemagne convoked the Council of Aachen (809) and decreed that this addition was a dogma necessary for salvation. With this fait accomplit under his belt, he tried to pressure Pope Leo III into accepting it.
Pope Leo rejected the filioque not only as an addition to the Creed, but also as doctrine, claiming that the Fathers left it out of the Creed neither out of ignorance, nor out of negligence, nor out of oversight, but on purpose and by divine inspiration. What Leo said to the Franks but in diplomatic terms, was that the addition of the filioque to the Creed is a heresy.
The so-called split between East and West was, in reality, the importation into Old Rome of the schism provoked by Charlemagne and carried there by the Franks and Germans who took over the papacy.
The Franks exercised a very deliberate policy of provoking doctrinal differences between the East Romans (the Orthodox) and the West Romans (the Roman Catholics). This policy was adopted “in order to break the national and ecclesiastical unity of the original Roman nation.” Romanides again,
“Because of this deliberate policy, the filioque question took on irreparable dimensions. However, the identity of the West Romans and of the East Romans as one indivisible nation, faithful to the Roman Christian faith promulgated at the Ecumenical Synods held in the Eastern part of the Empire, is completely lost to the historians of Germanic background, since the East Romans are consistently called "Greeks" and "Byzantines."
Thus, the historical myth has been created that the West Roman Fathers of the Church, the Franks, Lombards, Burgundians, Normans, etc., are one continuous and historically unbroken “Latin” Christendom, clearly distinguished and different from a mythical “Greek” Christendom. The frame of reference accepted without reservation by Western historians for so many centuries has been “the Greek East and the Latin West.”
In 879 AD, the Eighth Ecumenical Council condemned all those who would either add to or delete anything from the Nicene Creed. It is clear from the correspondence of St. Photios in the East with the Western Pope John VIII, that the Pope joined in condemning the filioque.