Spanish bishop of Córdoba who, as ecclesiastical adviser to Emperor Constantine I, was one of the chief defenders of orthodoxy in the West against the early Donatists (q.v.).

Consecrated bishop of Córdoba (c. 295), Hosius attended the Council of Elvira (Granada, c. 300) and from 312 to 326 acted as ecclesiastical adviser at the court of Constantine, who in 324 sent him as imperial emissary to the East to settle the Arian dispute (see Arianism). Hosius convoked a synod at Alexandria of Egyptian bishops and another at Antioch of Syrian bishops, at both of which Arius and his followers were condemned.

Prompted by Hosius, Constantine then summoned the first ecumenical Council of Nicaea (Nicaea, Council of) (325), where Hosius was influential in securing the inclusion in the Nicene Creed of the key word homoousios (see homoousian (homoousios)). In 342/343, he presided at the Council of Sardica (Sofia), which the Eastern bishops boycotted because the Westerners insisted on the presence of Bishop St. Athanasius the Great of Alexandria, a principal opponent of Arianism.

From 353 to 356 Hosius strongly resisted the efforts of the Arian emperor Constantius II to have Athanasius condemned by the Western bishops and in a famous letter reproved Constantius for intruding into ecclesiastical matters. Summoned to Sirmium in 356 and detained at court for a year, after threats and physical violence, Hosius signed the Arian formula of Sirmium (357) but retracted his signature before he died.