Consecrated bishop of Córdoba (ca.295), he narrowly escaped martyrdom in the persecution of Maximian (303-305). In 305 or 306 he attended the council of Elvira (Granada), and upheld its severe canons concerning such points of discipline as the treatment of those who had abjured (lapsed) their faith during the recent persecutions, and questions concerning clerical marriage.
From 312 to 326 acted as ecclesiastical adviser
/ privy counsellor at the court of the holy emperor St [[Constantine the Great]], who in 324 sent him as imperial emissary to the East to settle the Arian dispute. 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.
The saint also advised St Constantine to convene the [[First Ecumenical Council]] at Nicea in 325, where Hosius was influential in securing the inclusion in the [[Nicene-Constantinopolitan Creed|Nicene Creed]] of the key word ''[[homoousios]]''. He was the first to sign the acts of this Council.