Filioque

The filioque isn't a heretical addition to the Faith, it is a non-Byzantine expression of the Faith.

Historically, it was introduced to combat an Arian heresy professed in the West, on the Hispanic peninsula. The Latin translation of the Creed as formulated at Constantinople I in 381 AD (which was in itself a reaction to a heresy professed in the East) was amended, for the Latin translation didn't convey the same implications as the Greek original. Which is why Catholics don't say the filioque when reciting the Creed in Greek.

Theologically, it's an authentic (albeit non-Byzantine, non-Antiochian, and non-Photian) rendering of the Apostolic faith, as can be seen even with the Eastern fathers who profess the procession of the Spirit through the Son. Which is exactly what the filioque is meant to convey.

Byzantines and Romans, Catholics and Orthodox held communion even after the formal introduction of the filioque in the 6th century AD. It wasn't until after the Great Schism of 1054 that it became such a bone of contention.