bishop of Dorylaeum and famous opponent of the Nestorians (Nestorian) (who believed that the divine and human persons remained separate in Christ). He was one of the formulators of doctrines at the ecumenical Council of Chalcedon (451).

While a layman, Eusebius was the first to challenge publicly (429) the teaching of Patriarch Nestorius of Constantinople, posting throughout Constantinople his famous Contestatio, summoning the faithful to rise against Nestorius. His action led to Nestorius' condemnation by the Council of Ephesus (431).

In 448 Eusebius, then bishop of Dorylaeum, charged his friend Eutyches, an archimandrite at Constantinople, with heresy for holding a doctrine later known as monophysitism (which asserted that Jesus Christ had but one nature, not two). The charge by Eusebius, the first to oppose monophysitism, led to Eutyches' deposition by a synod summoned by Bishop Flavian of Constantinople. Eutyches was then excommunicated by Pope Leo I the Great but was reinstated by action of the council that met in Ephesus (known as the Robber Synod of Ephesus) in 449; Eusebius was deposed for his role in the matter. He immediately appealed to Leo and was given refuge in Rome. In 451 he was rehabilitated by the Council of Chalcedon, for which he assisted in drafting the classic definitions of the person and natures of Christ and which caused Eutyches' banishment.