The Greeks and the Latins, during the sixth session (July 6, 1439) Council of Florence, were able to sign this common definition:
“In the name of the Holy Trinity, Father, Son and Holy Spirit, with the approval of this sacred and universal Council of Florence, we establish that this truth of faith must be believed and accepted by all Christians and thus all must profess that the Holy Spirit is eternally of (the Father and the Son), that he has his existence and his subsistent being from the Father and the Son together, and that he proceeds eternally from the one and from the other as from a single principle and from a single spiration.”
There is an additional clarification to which St. Thomas had devoted an article of the Summa (Ultrum Spiritus Sanctus Procedat a Patre Per Filium).
“We declare,” said the Council, "what the holy Doctors and Fathers stated that is, that the Holy Spirit proceeds from the Father through the Son tends to make understandable and means that the Son too, like the Father, is the cause, as the Greeks say, and the principle, as the Latins say, of the subsistence of the Holy Spirit.
And since all that the Father has he has given to the Son in his generation, with the exception of being Father, this very procession of the Holy Spirit from the Son the Son himself has eternally from the Father, from whom he has been eternally generated."
At the conclusion of the council, the reunion of the Eastern and Western Church was (joyfully proclaimed).
This joyful reunion was not to last long, however.
The military aid that the east hoped to gain from the west (the east was struggling with the conquering Turks) never materialized and the eastern monks rejected the decisions of the council.
The eastern Church has always maintained that the insertion of the word filioque into the creed was illegal. However, the Apostle’s Creed (the ‘Creed of Creeds) was never endorsed or accepted by any council or synod(and this creed is universally recognized, although it is used less in the east.
Many creeds were only approved by local councils (or synods), and most were in use in the churches before approval.
Further, the Nicene creed itself was in use hundreds of years (and had undergone changes) before being recognized by a truly ecumenical council (the Council of Chalcedon, 451).
Even so, while the east accuses the west of changing the creed approved by the Council of Chalcedon, they themselves changed the creed at the second Council of Nicaea by adding (through the Son).
After the Council of Florence the West continued to profess that the Holy Spirit “proceeds from the Father and the Son,” while the East continued to hold to the original formula of the Council of Constantinople.
Although this change never took hold in the eastern churches.
Doctrinally, I believe that the teaching of the Bible is best summarized by Augustine:
“The Son, is born of the Father, and the Holy Spirit proceeds from the Father principally and (by [the Father’s] gift and with no lapse of time) commonly from both”.
Further, I claim that the word (filioque) was lawfully inserted into the creed by the Council of Florence, (a truly ecumenical council) that reunited the East and the West (if only for a short time).
That the eastern monks rejected the decisions of the council is not relevant, it only demonstrates their disrespect for authority.
Also, there are certain considerations which form a direct proof for the belief of the Greek Fathers in the double Procession of the Holy Ghost.
First, the Greek Fathers enumerate the Divine Persons in the same order as the Latin Fathers; they admit that the Son and the Holy Ghost are logically and ontologically connected in the same way as the Son and Father.
Secondly, the Greek Fathers establish the same relation between the Son and the Holy Ghost as between the Father and the Son; as the Father is the fountain of the Son, so is the Son the fountain of the Holy Ghost.
But since the time of the Second Vatican Council a fruitful ecumenical dialogue has been developing.
It seems to have led to the conclusion that the formula (Filioque) does not constitute an essential obstacle to the dialogue itself and to its development, which all hope for and pray for to the Holy Spirit.
Union is still possible on the “Filioque” issue through the recognition that the formulas “and the Son” and “through the Son” mean the same thing so long as future members of the Orthodox Church do not use this as an excuse like Photius did AD 882.