Historical origin: Separation of Carthaginensis
The Diocletianic provinces of Hispania
Originally, after the provincial division by Diocletian in the 3rd century, the city of Toletum (now Toledo) was within the Roman province of Carthaginensis, whose capital was Carthago Nova (now Cartagena). Hispania's division into ecclesiastical dioceses was based on the Roman provincial divisions, so that the episcopal seat at Toledo originally a dependent of the seat at Cartagena.
The problem arose in the mid 6th century when the Byzantine emperor Justinian seized control of an important strip of Hispania, including diocesan seats as important as Cartagena (renamed Carthago Spartaria by Justinian), Corduba,
Begastri and Illici. The metropolitan seat and provincial capital were in the territory occupied by the Byzantines and so, shortly after taking the throne, the Visigoth king Gundemar promoted the holding of a synod in Toledo. That synod agreed that Toledo was the metropolitan of the whole province, seizing that title from the episcopal seat at Cartagena - that agreement was then endorsed by the king in a decree of 23 October 610.
The Islamic invasion in 711 left Toledo as a city under Islamic rule for the next 250 years, first way away from the frontier by the geographic boulder of the Duero river in the northern high plains, and later by the Central Sierras, as the frontier itself Taifa realm for another 80 years where contacts with the Christian kingdoms north would resume closer. That position left the See under Islamic suzerainty, tolerated as a heretic church of Allah but subject as Dhimmi or client as Jews were too, what would make it be respected but seen by Christian kings and their bishops with suspicion on its policies, influenced by the policies of Cordoba. After the re-conquest Toledo was held or contested by the Kingdom of Castille and the Kingdom of León and sometimes as
Infantazgo by semi-independent queens of the royal house. During the Reconquista, the alliance between the monarchs and the church concentrated on the distinct privileges that one offered the other. Soon after Alfonso VI's conquest of Toledo, the pope issued the bulla Cunctis Sanctorum in 1088/1089, recognising the holders of the diocese of Toledo as "primates" and "metropolitans" as they had been during the Visigothic era, and dubiously and unofficially under Islamic rule.