Vol. 1: A seculo VI usque ad 803

Pope Pelagius II writes to Helias, the patriarch of Aquileia, and all the bishops of Istria, the province whose prelates still persevered in their refusal to condemn the three chapters denounced by the Fifth Ecumenical Council (Constantinople II) of 553, urging them to desist from their schismatic ways and return to the embrace of the Catholic Church.

Lombard King Authari dispatches an army led by duke Ewin of Trento to attack Istria, at the time a Byzantine province. Following successful Lombard attacks, a one-year peace is negotiated between the warring parties (narrative account from Paul the Deacon's History of the Lombards). 

Following the death of Aquileian patriarch Helias, his successor, Patriarch Severus, together with three other bishops (including the bishop of Poreč, John) are imprisoned in Ravenna by the Byzantine exarch Smaragdus; in order to buy their freedom, the prelates agree to condemn the Three Chapters and reunite with Rome, a move that greatly angers other bishops in the ecclesiastical province of Aquileia. Finally, a synod is held in Marano (591) whereby Patriarch Severus is forced to formally acknowledge the error of his ways in supporting the condemnation of the Three Chapters (narrative accounts from Paul the Deacon's History of the Lombards).

Romanus, the exarch of Ravenna, writes to King Childebert II of Austrasia, informing him of his triumphs against the Lombards and asking him for military aid; Grasulf, the Lombard duke of Friuli, also supported Romanus' military efforts and he received Romanus' aid in his fight against his enemies in Istria.

Pope Gregory I writes to Severus, the patriarch of Aquileia, requesting on behalf of the emperor (Maurice) that he come to Rome with his clergy and be judged on a synod (for his unwillingness to condemn the Three Chapters and his perseverance in supporting the schism).

The bishops of the ecclesiastical province of Aquileia write to emperor Maurice, beseeching him not to allow their metropolitan, Archbishop Severus of Aquileia, who was forcefully taken to the pope in Rome, to be put on trial and judged by the adversaries of the Three Chapters, that is, the non-schismatic clergy; the bishops expound their viewpoints on the theological controversy, their fealty to the Ecumenical Council of Chalcedon, and why they refuse to denounce the so-called Three Chapters condemned by Emperor Justinian I and the Ecumenical Council of Constantinople in 553.

Emperor Maurice writes to Pope Gregory I, urging him not to treat violently the schismatic bishops of the ecclesiastical province of Aquileia (the "Istrian bishops") and not to compel them to journey to Rome.

The regions of Ravenna, Grado, and Istria are ravaged by the so-called Justinianic plague as Agilulf, King of the Lombards, makes peace with the Avars (narrative accounts from Paul the Deacon's History of the Lombards).

Pope Gregory I writes to Callinicus, the Exarch of Italy, regarding a variety of affairs, including the recent military victory over the Slavs and the return of the Church of Koper to the Catholic creed.

Pope Gregory I writes to Maxim, the bishop of Salona, on a variety of matters, including the latest advances of the Slavs and their recent incursions into Italy by way of Istria.

Perpetual peace is celebrated between the Lombard King Agilulf and the Avar Khagan as Byzantine Istria is attacked by the combined forces of the Lombards, Avars, and Slavs (narrative account from Paul the Deacon's History of the Lombards).

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.

Lombard King Agilulf renews peace with the Byzantine Emperor and the Franks as the Slavs ravage across Istria (narrative accounts from Paul the Deacon's History of the Lombards).

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.

Pope John IV dispatches abbot Martin to journey across Dalmatia and Istria to ransom the captives (imprisoned, one would assume, by the still pagan Slavs) and return the relics of the saints from these endangered places to Rome; narrative account from the Book of the Popes (Liber pontificalis).

De terminatione provinciarum Italiae – Istria together with Venice featured as the first region of Italy in an early medieval catalogue of the provinces of Italy.

Ansfrit of Ragogna rebels against Rodoald, the Lombard duke of Friuli, and conquers his duchy as Rodoald flees to Istria from where he sails to Ravenna and finally reaches King Cunipert in Pavia (narrative account from Paul the Deacon's History of the Lombards).

Under Lombard King Cunipert, the Synod of Pavia takes place where the representatives of the schismatic clergy of the ecclesiastical province of Aquileia officially accept the decrees of the 5^th Ecumenical Council (Constantinople II) and finally desist from their schism; narrative account from the contemporary "Poem on the Synod of Pavia" (Carmen or Rythmus de Synodo Ticinensi).

Istria as described in the chapters of Cosmography, penned by the anonymous Ravenna Cosmographer (Anonymus Ravennas), including the region's rendition on the famous Peutinger's Map (Tabula Peutingeriana).

Pope Gregory II writes to Aquileian patriarch Serenus, reminding him that he received the pallium at the behest of the Lombard king and that he ought to exercise his ecclesiastical rights within the confines of Lombard territories, not interfering with the jurisdictions of the Patriarchate of Grado.

Pope Gregory II writes to Donatus, the patriarch of Grado, to Duke Marcellus of Venice, and to the clergy and populace of Venetia et Histria, informing them that he has heard their lamentations regarding the actions of the “Friulian bishop” (the patriarch of Aquileia) and that he has undertaken appropriate measures, ordering the invaders to remain within the confines of their jurisdictions.

Pope Gregory II writes to the bishops and populace of Venice and Istria, notifying them that Peter, the former bishop of Pula who usurped the Patriarchate of Grado following the death of Patriarch Donato, has been removed and returned to his See in Pula and that the bishops ought to elect a new head of the church of Grado.

Pope Gregory III writes to archbishop Anthony of Grado and to his suffragans, inviting them to the church synod to be held in Rome in the coming November where the issue of icon veneration will be discussed.

Acts of the 731 Synod of Rome whereby the division between the Patriarchate of Grado and the “Bishopric of Friuli” (Patriarchate of Aquileia) is sanctioned, officially confirming Grado as “New Aquileia” and the metropolitan see of the entire ecclesiastical province of Venetia et Histria; 11^th-century forgery.