⭐ A local council in Spain changed one line of the creed of the whole church — five hundred years later, East and West excommunicated each other over it.
The issue
Does a local synod have the right to change a creed passed by an ecumenical council?
The ruling
⚠ The council formally added the filioque (and the Son) to the Latin creed of the West. Olson records that the only thing that can be known for certain is that the clause was formally added to the Latin creed in the West by the Spanish synod held at Toledo in 589; before that synod, he adds, the clause was already common among monks in the West.
Aftermath
⚠ Notice the shape of this event on this thread: no one ever made a decision that said, we have the right to change the creed. The clause first spread among monks, then a local council confirmed it, and then it became the usual version in the West — by the time the East noticed, it was already, as Olson puts it, widely accepted as orthodox doctrine. ⭐ Authority is sometimes not claimed; it is assumed by default.
⚠ Tracing it afterwards is actually hard. Olson admits that in the whole filioque controversy the actual facts — when, where, and who did what — are wrapped in mystery and the historical record is unclear, and that the accounts of Eastern church historians differ sharply from those of Western ones. Some have guessed that Augustine himself came up with the idea; Olson judges this unlikely — though Augustine, in On the Trinity, did indeed assert that the Holy Spirit proceeds from the Son.