When Rome fell, the Visigoths already turned christians (even if they followed the Arianism branch) and decided to keep the administrative demarcations of the Romans instead of changing the whole system. That being said, the oldest evidence of a bishopric with Girona as its capital (at the time called Gerunda), dates from the 6th century AD. From then on, the bishopric grew in importance. But ultimately, Barcelona’s bishop gained more political relevance.