Eusebius of Caesarea
Bishop of Caesarea, author of the first history of the church and of a Life of Constantine. He took part in the Council of Nicaea, where he had at first sided with Arius’s supporters, and signed its creed.
Eusebius was bishop of Caesarea in Palestine. His Church History (Church History) is a principal source for the first three centuries of Christianity, and his Life of Constantine (Life of Constantine) for the emperor’s dealings with the church.
Before the First Council of Nicaea Arius counted him among the bishops on his side (Theodoret, Church History 1.4). Eusebius attended the council and describes its opening (Eusebius, Life of Constantine 3.6–14). Afterwards he wrote to his own church to explain why he had signed a creed containing the word homoousios, after questioning what it meant (Socrates, Church History 1.8); Athanasius later cited the letter against those who rejected the creed (Athanasius, Defence of the Nicene Definition 3). He died a few years after Constantine and was succeeded by his pupil Acacius (Socrates, Church History 2.4).
In the timeline
- May AD 325, firm dateThe First Council of Nicaea
Constantine gathers some three hundred bishops at Nicaea over the teaching of Arius. They confess the Son to be “of one substance with the Father”, agree a common date for Easter and issue twenty canons. Catholic, Orthodox and Reformation churches still appeal to the council.