Pope Pelagius II confirms Grado as the new metropolitan see of the ecclesiastical province of Venetia et Histria; 11th-century forgery.
The acts of the synod of Grado, heavily interpolated by later falsifications, by which the bishops of the ecclesiastical province of Aquileia remain faithful to the Catholic creed as decreed by the Ecumenical Councils of Chalcedon (451), Ephesus (431), Constantinople I (381) and Nicaea (325), refusing to denounce the Three Chapters condemned by the Second Ecumenical Council of Constantinople (552).
Following the death of Aquileian patriarch Severus, two new patriarchs are ordained: John, supported by the Lombards and with his seat in Aquileia; and Candidianus, supported by the Romans (Byzantines) with his seat in Grado - the definitive and official split of the patriarchate of Aquileia into two patriarchal sees (narrative account from Paul the Deacon's History of the Lombards).
Patriarch John of Aquileia writes to Lombard King Agilulf, complaining that the ill-ordained Candidianus was consecrated as the bishop of Grado by the three Istrian bishops who were violently forced to do so by the “Greeks” (the Romans, that is, the Byzantines), and he beseeches the king not to allow such consecrations to continue following Candidianus' death.
Pope Honorius incites all the bishops of the ecclesiastical province Venetia et Histria to profess disobedience to the deposed patriarch Fortunatus of Grado and to accept subdeacon Primogenius, to whom the pope had already sent the pallium, as their new lawful head of the ecclesiastical province; moreover, the pope promises to open diplomatic negotiations with the Lombard king, with whom the deposed Fortunatus sought refuge, with the goal of restoring all the ecclesiastical goods stolen by the said Fortunatus.
Emperor Charlemagne bestows immunities upon the Church of Grado due to the special services and merits of its incumbent Patriarch Fortunatus II.
Emperor Charlemagne exempts the four ships of Fortunatus II, "the patriarch of the Venetians and Istrians," from all the tolls.
The Plea of Rižana (Placitum Rizianense): The representatives of Istrian towns and cities present their grievances to counts Aio and Cadulus, the envoys of the Roman emperor Charlesmagne, in an official placitum held by the river Rižana in the district of Koper.
Emperor Louis the Pious promises to Patriarch Fortunatus II and to all the Istrians that their right to elect their own patriarchs, bishops, abbots, tribunes, and other officials will be respected and that the rulings of the judicial assembly (placitum) held by Rižana will be upheld.
Emperor Louis the Pious and his son Lothair write to Venerio, the patriarch of Grado, confirming the possessions of his Church, including the Patriarchate's jurisdictions in Istria.
The decrees of the Synod of Mantua: the long conflict between the patriarchs of Grado and Aquileia over the metropolitan jurisdiction over Istrian bishoprics is settled in favor of the Aquileian Church.
A poem composed in response to the Synod of Mantua (6th of June, 827), arguing that Aquileia, the old metropolitan see of the ecclesiastical province Venetia et Histria, was so thoroughly destroyed by barbarians (Attila, the Avars, Lombards and Goths) that it was never rebuilt; thus, the metropolitan see transferred to "the Venetians" and Aquileia was demoted to a status of a mere parish.
Emperor Louis II confirms the charter of Lothair I to the Patriarchate of Aquileia regarding the dispute with the Patriarchate of Grado that was settled at the Synod of Mantua in 827: the patriarchs of Aquileia are to enjoy the metropolitan jurisdiction over all the Istrian bishoprics.