Marduk, by “constant orthodoxy”, do you mean the CC always believed in the filioque?
Alhough one might claim that it has been believed from the beginning, there is no evidence of it.
The filioque was introduced like a gloss, into the creed of Spain, which was at the time overrun with Arian heretics, The purpose was to try to reinforce the idea that Jesus Christ was divine by showing that He, like the Father, was a source of the Holy Spirit and thus
had to be divine to do that.
This happened, not at an ecumenical Council, but at a regional Council (which the western Catholic church used to have in the early days) in the year 589AD, which is about 550 years after the Pentecost event.
At that point, the filioque was recited among the Catholics of Spain, but no where else.
Why this was significant, was because in those days the Catholic church also had different Masses (liturgies) from region to region. So, the local churches called their own Councils, and had different liturgies, all across Europe.
In Spain the Nicene Creed was recited in the liturgy (an early version of the Mozarabic liturgy) in the same manner as the orthodox eastern churches. This was not true of the liturgy in Gaul (France) or the liturgy in Italy, or even in the city of Rome. The Mass did not include a recitation of the Creed, and the Creed taught was the original version.
King Charlemagne’s church at his capital adopted the practice of reciting the Nicene Creed in 798AD, about 200 years after the Spanish/Mozarabic church. They chose to use the version of the creed used in Spain (possibly because he controlled a small part of Spain in his day). It spread from Aachen all over the region under Charlemagne’s control (the Frankish kingdom was quite extensive). Charlemagne was aware (or became aware) of a difference in his version of the Creed from the eastern version, because he actually accused to Greeks of omitting or dropping the filioque (it was probably a political ploy). He seems to have been unaware of the actual history of the phrase in the creed.
Pope Leo, the Pope at the time, opposed the use of the Creed with the filioque, but in the Catholic church of those days he had no authority to make either the Spanish church or the church in Gaul change their practices in the liturgy. He may have been under some pressure from Charlemagne to conform, because he manfully opposed the inclusion of the filioque in the creed within his own jurisdiction.
It was not until the year 1014AD that the Creed was recited in Rome, including the filioque. This was 425 years after it was introduced by the Mozarabic church of Spain, 623 years after the Council of Constantinople forbad any changes to the creed. It was after this event (1014) that the Pope was dropped from the dyptichs of the eastern Catholic churches.
It was still another two hundred years before the filioque was made a dogma of the Roman Catholic church. Thus over 600 years between it’s introduction in Spain to it’s confirmation as a dogma. From the Catholic Encyclopedia …
The rejection of the Filioque, or the double Procession of the Holy Ghost from the Father and Son, and the denial of the primacy of the Roman Pontiff constitute even today the principal errors of the Greek church. While
outside the Church doubt as to the double Procession of the Holy Ghost grew into open denial, inside the Church the doctrine of the Filioque was declared to be a dogma of faith in the
Fourth Lateran Council (1215), the Second council of Lyons (1274), and the Council of Florence (1438-1445). Thus the Church proposed in a clear and authoritative form the teaching of Sacred Scripture and tradition on the Procession of the Third Person of the Holy Trinity.