When the "Church of Olbia and Gallura" considers its own origins it finds Saint Simplicius, bishop, and his companions of martyrdom: Rosula, Diocletianus and Florentius. According to the local tradition, they are the first witness of Christian faith in the North-East of Sardinia. News about them are found in local traditions, in the Passions and in the martyrologies. The most ancient source are the martyrologies or rather the books of the anniversaries of the martyrs that are object of commemoration in a local Church; the martirologies had as their source the liturgical calendars of the local Churches, in use still before the Constantinian peace (313 A.D.). Saint Simplicius and his companions martyrs are remembered in numerous martirologies, in first place in the one called Geronymianum, compiled in 431-450; in it there was a "laterculus" or inscription regarding the anniversary of the martyrdom of Saint Simplicius and his companions.
We have reconstructed it in the following way: "On the 15th of May, in Sardinia, in the city of Fausiana, people celebrate the anniversary of the martyrdom of Saint Simplicius, priest, and in the same city of Fausiana, the anniversary of the martyrdom of Saint Rosula, and of the Saints Diocletianus and Florentius." From the "laterculus" two datas emerge, the place of death (or of the tomb): "Fausiana"; the feast-day of the anniversary: "the 15th of May." Insofar Saint Simplicius cult has taken root in a well precise place and it is tied to a well specific local Church: Fausiana. What do we know of it? Fausiana was, beginning from 431-450 and then particularly in Byzantine epoch, the diocese of northern eastern Sardinia, that gave origin to the kingdom or Giudicato of Gallura.
We know the name of only one of its bishops, Victor, who was active between the end of the VIth century and the beginning of the VIIth century. Pope Gregory the Great entrusted to him, in the year 555 A.D., the evangelization of Gallura and of Barbagia populations. The diocese of Fausiana was, probably, the prosecution of the "Church of Olbia", in full vitality already before Constantinian peace (313 A.D.) Sardinian scholar Damiano Filia argues that Olbia' paleo-christian cemetery (situated on Saint-Simplicius-hill) is a clear sign that the city has been evangelized during the first three centuries of Christian era and that such cemetery is to consider as memory of an ancient Christian centre. Such a centre is identified by him with Fausiana, of uncertain site but next to the roman-founded-Olbia and outside the city walls. The same author thinks also that Saint Simplicius has been Fausiana first bishop. In the kingdom or Giudicato of Gallura, initially there was only one diocese, recorded on 1095 as the "episcopatus Gallurensis"; and it would have been the prosecution of that of Fausiana. Between 1050/1070 and 1120/1125 there was built, perhaps on the foundations of a more ancient one, Saint Simplicius church, probably as the episcopal see of the diocese of Gallura and then of that of Civita.
All makes to suppose that Saint Simplicius church has been built on the sepulchre ("locus depositionis") of Olbia' martyrs, whose relics are still preserved. On the 8th of December 1614 the relics of the martyrs Simplicius, Rosula, Diocletianus and Florentius were transported, by an order of Diego Passamar, bishop of Ampurias and Civita, in Saint Paul' parish church, the main parish of Olbia, called then Terranova. Saint Paul's was then inside the city walls; a place, therefore, more suitable for their maintenance. On the same day bishop Diego Passamar found the relics of Saint Aurelia Florentia, a Olbia' four years old girl, who died during the Vandalic persecution of Olbia' Catholic communitiy. This historical research has been destined since the beginning, in the intentions of the promoters and in those of the author, not only to Olbia' people and Gallura' populations, but also to the tourists that, more and more, come to visit Olbia' Saint-Simplicius-cathedral-church. We tried to put together all the news we found about the following themes: the origins of Christianity in Olbia and in Gallura; the cult of Saint Simplicius and companions martyrs; the organization of the diocese of Fausiana, of Gallura, of Civita; the churches dedicated to Saint Simplicius; Olbia' Saint-Simplicius-cathedral-church, today being a pontifical "basilica" by a decree of the Sovereign Pontiff John Paul IInd; the parish organization in the diocese of Civita (ancient Olbia); the names list of Civita bishops (included those of Ampurias, diocese to which Civita was united in 1506).