noted supporter of St. Athanasius the Great (Athanasius, Saint) of Alexandria, Egypt, and restorer of the Nicene Creed, the orthodox doctrine adopted by the first Council (325) of Nicaea, which declared the members of the Trinity to be equal.

Eusebius became the first bishop of Vercelli in 345. Living in community with his priests, he was the first Western bishop to unite monastic life with the ministry. As the emissary of Pope Liberius (Liberius) at the Council of Milan (355), he refused to sign the condemnation of Athanasius for his attacks against Arianism. For supporting Athanasius, Eusebius was exiled to the East. Pardoned by the Roman emperor Julian the Apostate (Julian), he attended the Synod of Alexandria (Alexandria, Synod of) (362), whose decrees on the Nicene Creed he promulgated, thereby helping to restore orthodoxy and unity throughout the empire. Returning to Italy, he worked with St. Hilary, bishop of Poitiers, France, in opposing Arianism.

Three letters written during his exile are extant. The first seven books of De Trinitate, long attributed to Athanasius or Bishop Vigilius of Thapsus, are presently accepted as Eusebius' work.