As I was reading up on the first two Ecumenical Councils, I came across a fascinating tidbit of information from Protestant historian Philip Schaff's famous Nicene Fathers series. For those who don't know, the Nicene Creed we recite each Sunday actually came to us from two Ecumenical Councils. Basically, the First Ecumenical Council held in Nicaea in AD325 gave us the 'first half' of the Creed, up to the words "and we believe in the Holy Spirit," but stopped there. Later on at the Second Ecumenical Council held in Constantinople in AD381, we got the 'second half' of the Nicene Creed, which added everything including and after the words "and in the Holy Spirit, the Lord and giver of life, who proceeds
from the Father". What is fascinating is that it turns out these Creeds were not just invented on the spot at these Councils, but rather they existed in a few different 'versions' and were basically used as a 'statement of faith' for one's Baptism. This is an important historical detail because it means that the Filioque - the part where the Creed says the Holy Spirit proceeds from the Father "and the Son" - despite not being part of the Nicene Creed of either the Council of 325 nor 381, should not be automatically taken as a rejection of the idea itself. Nor should having the Filioque clause within the Creed be taken as 'tampering' with the Creed. (I wrote about the Filioque taught in Scripture in an older post, if you're interested.)
The best testimony for this comes from a significant Early Church Father, St Epiphanius, Bishop of Salamis (Cyprus). He lived from AD310-403 and is held in high regard by both East and West. He lived during both Councils but he did not attend either, making his testimony even more significant. The passage from Schaff's series says that Epiphanius used a creed as early as AD374 (i.e. a decade prior to the Second Ecumenical Council), which none the less was nearly identical to the Nicene Creed as we know it. Epiphanius tells us that this was handed on from ancient times, even from the Apostles themselves, and that it is required knowledge to get Baptized. This means that prior to the Second Ecumenical Council in AD381, certain regions were already using a longer Creed than the one from AD325. Yet we would not say these regions were 'tampering' with the Creed, since they were expounding on it without changing its meaning.