Eusebius of Nicomedia, d. 342, Christian churchman and theologian, leader of the heresy of Arianism. He was bishop of Nicomedia (330-39) and patriarch of Constantinople (339-42); Eusebius was powerful because of his influence with Roman Emperor Constantine I and particularly with the emperor's son, Constantius II. He sheltered Arius in 321 and fought his condemnation at Nicaea (see Nicaea, First Council of). Eusebius signed the Nicene formulary but was exiled by Constantine shortly afterward. Eusebius' influence on the emperor's sister Constantia, however, soon won him his reprieve (328). As adviser to Constantius, a committed Arian, he systematically advanced a moderate Arianism throughout the empire.