Hila'rius Diaconus or Hila'rius Diaconus
surnamed DIACONUS, a native of Sardinia, a deacon of the church at Rome in the middle of the fourth century, and hence designated Hilarius Diaconus,
to distinguish him from others of the same name, was deputed by Pope Liberius, along with Lucifer of Cagliari, Eusebius of Vercclli, and Pancratius, to plead the cause of the orthodox faith before Constantius at the council of Milan. Upon this occasion he defended the principles of Athanasius with so much offensive boldness, that he was scourged by order of the emperor, and condemned to banishment, along with his companions. Of his subsequent history we know little, except that he adopted the violent opinions of Lucifer to their full extent, maintaining that not only Arians, but all who had held any intercourse with them, as well as heretics of every description, must, even after an acknowledgment of error, be re-baptized before they could be admitted into the communion of the Catholic church, and from this doctrine he was sarcastically styled by Jerome a second Deucalion.
Two treatises are sometimes ascribed to this Hilarius, both of very doubtful authenticity. One of these, Commentarius in Epistolas Pauli,
has frequently been published along with the writings of Ambrosius; the other, Quaestiones Veteris et Novi Testamenti,
among the works of Augustin.