What is Filioque
Filioque is a Latin word which translates as 'and the son'.
The Filioque Clause are the three words added to the procession of the Holy Spirit in the Nicene Creed.
We believe in the Holy Spirit
the Lord, the giver of life
who proceeds from the Father and the Son
who with the Father and the Son
is worshiped and glorified
In 325 AD the Oecumenical Council of Nicaea endorsed a common creed for the Church.
In 381 AD the Oecumenical Council of Constantinople extended and refined the Nicene Creed.
On both these occasions the Creed did not include the Filioque clause.
In 431 AD the Oecumenical Council of Ephesus affirmed the Nicene Creed and declared anathemas on those who would add to or take away from the Creed.
In 451 AD The Oecumenical Council of Chalcedon Affirmed the Three previous Councils, and went to some length to expand on the Christology of the Church, but did not change the Creed, which still did not have the Filioque clause.
In 794 AD the Council of Frankfurt (a small council called by Charlemagne), against the advice of the Pope argued to include the Filioque to combat Spanish Adoptionism, or to shore up the distinctions between the Holy Roman Empire and the Byzantine Empire.
In 809 from the meeting at Aix-la-Chapelle, Charlemagne sent emissaries to the Pope for permission to use the Filioque. The Pope said no, and the FGrench continued to say it.
In 1014, to secure the support of Henry 2 (HRE) Benedict IX (Pope) allowed the filioque to be said in Rome, which led to the division of the Church in 1054 - now called the Great Schism.
To some extent the issue rested on the question of who has the authority to change the Creed - The whole Church in Oecumenical Council, or the Pope in his own right.