Ursicinus, also known as Ursinus, was elected pope in a violently contested election in 366 as a rival to Pope Damasus I. He ruled in Rome for several months in 366–367, was afterwards declared antipope, and died after 381.
Pope Liberius had been banished in 355, as a result of a conflict with the Emperor Constantius II over the treatment of Arianism. Antipope Felix II was imposed as his successor. After the Emperor's death, Liberius was eventually reinstalled and Felix expelled from Rome. The rival parties remained highly polarized in Rome, however.
Liberius died on 24 September 366. In the early Church, new Bishops of Rome were chosen in the manner customarily used in the other dioceses, that is, the clergy, with the people of the diocese, elected or chose the new bishop in the presence of the other bishops in the province. This was a simple method in a small community of Christians that was unified by persecution.
But as the Christian congregation of Rome grew in size, the acclamation of a new bishop was fraught with division. Rival claimants and a certain class hostility between patrician and plebeian candidates unsettled some episcopal elections. At the same time, 4th century emperors expected to confirm each new pope.
The upper-class partisans of Felix supported the election of Damasus, but the opposing supporters of Liberius, the deacons and laity, supported Ursicinus; the two were elected simultaneously, in an atmosphere of rioting. Supporters already clashed at the beginning of October. Such was the violence and bloodshed that the two praefecti of the city were called in to restore order, and after a first setback, when they were driven to the suburbs and a massacre of 137 was perpetrated in the basilica of Sicininus (as cited in Ammianus Marcellinus), the prefects banished Ursicinus to Gaul. There was further violence when he returned, which continued after Ursicinus was exiled again.
Church historians, such as Jerome and Rufinus, took the part of Damasus. At a synod in 378 Ursicinus was condemned and Damasus exonerated and declared the true pope. The former antipope continued to intrigue[vague] against Damasus for the next few years, and unsuccessfully attempted to revive his claim on Damasus's death. Ursicinus was among the Arian party in Milan, according to Ambrose (Epistle iv).
A decree of 502 under Pope Symmachus ruled that laymen should no longer vote for the popes and that only higher clergy should be considered eligible.
- Henry Wace, Dictionary of Christian Biography: as "Ursinus"; a balanced historical account, with a detailed quote from the impartial pagan historian Ammianus Marcellinus.