Visigothic Spain held its church councils at Toledo, and they were assemblies of the kingdom as much as of the church, issuing law alongside doctrine. The third of them, in 589, is the decisive one: King Reccared renounced the Arian Christianity his people had held for two centuries and brought the Gothic nobility and clergy with him into the Catholic faith of the Hispano-Roman population. The kingdom's two peoples were now one church, and the councils that followed legislated for it in detail.
The acts of that council also contain the earliest well attested use of the creed with the word filioque, the assertion that the Holy Spirit proceeds from the Father and the Son. It was added in a local setting to close off any Arian reading, and it spread from Spain into Frankish usage and eventually into the Roman rite. The Greek churches regarded the insertion into a creed agreed by an ecumenical council as illegitimate whatever its content, and it became the standing doctrinal grievance between east and west, argued over at Florence eight hundred years later.
The same councils legislated harshly against Jews, ordering forced baptism, removing children from families and setting conditions for converts, and that is part of the record too. Toledo also kept the old Iberian liturgy, later called Mozarabic, which survived the Roman standardisation and was given a permanent chapel in the cathedral around 1500 so that it would not be lost. It is celebrated there still.