The Modern Day Roman Empire!
The Roman State in a de jure way survived the fall of Constantinople and restructured itself in Madrid as the Hispanic Roman Empire during the period known as the Greek renaissance of Spain. Last Byzantine Roman Emperor Andreas Palaiologos decreed in his testament titled "Constantinopolitani Romanorum Imperatoris heres" that Fernando of Aragón of the Spanish Catholic Monarchs will receive the crown and title, privileges and responsibilities of Roman Emperor. The grandson of the Catholic Monarchs and the first naturally born Spanish Roman Emperor was Charles V. He was crowned by the Pontifix Maximus in Rome as just that and used the Byzantine double headed eagle as his shield which subsequently became Spains(Hispania) coat of arms as well.
(Coronation of Charles V by the Pontifix Maximus as Roman Emperor under the Byzantine-Roman flag)
the Catholic Church of Rome and the Pontifix Maximus received full sovereignty from the Byzantine Roman Emperor through the council of Florence but yet was also elevated as the official state church of the Roman Empire to the dismay of the Greek orthodox patriarch of Constantinople, 50 years before its fall. This Roman state/church organizational system was passed on to Spain by Byzantium.
In summary, Spain is the de jure Roman State and the Roman Catholic Church is still its official state religion. Now Spain just needs to start making this de-facto.
(Hispanic Roman Emperor with Roman Curia, Vatican City, Rome)