homoousios, in Christianity, the key term of the Christological doctrine formulated at the first ecumenical council, held at Nicaea in 325, to affirm that God the Son and God the Father are of the same substance. The First Council of Nicaea, presided over by the emperor Constantine, was convened to resolve the controversy within the church over the relationship between the persons of the Trinity. The council condemned Arianism, which taught that Christ was more than human but not fully divine. The use of homoousios (Greek: “of one substance”) in the creed produced at the council was meant to put an end to the controversy, although the influence of Arianism persisted in the church for centuries. In 381 Emperor Theodosius I summoned the second ecumenical council, the First Council of Constantinople, which developed and affirmed the earlier creed. The resulting Nicene Creed also contained the word homoousios and became the definitive statement of orthodox belief.