For nearly two centuries the Visigoths ruled Spain as Arians over a population that was Nicene. The two groups had separate clergy, separate churches and separate baptisms, and the division ran along the line between rulers and ruled, which made it a political fact as much as a doctrinal one. An Arian king could not be crowned by the church most of his subjects belonged to.
Reccared converted early in his reign and then had to bring his kingdom with him. The council at Toledo in 589 is where that was done in public. The king read a profession of faith to the assembled bishops, his Arian bishops subscribed and were received, and the acts record the nobility with them. The council issued disciplinary canons and directed that the creed be recited at every mass, which is a practical instrument: a doctrine said aloud weekly by everyone is harder to walk back than one filed in an archive.
The conversion held, and it changed what the Visigothic state was. Councils at Toledo became a standing institution, meeting repeatedly over the following century, legislating on church and civil matters together, and functioning as something close to an organ of government. Bishops acquired a formal role in ratifying kings.
It also had a darker consequence that the atlas should not pass over. The same series of Toledan councils produced increasingly severe legislation against the Jewish population of Spain, and a unified Christian kingdom turned out to mean a kingdom with much less room in it.