homoousios, inChristianity, the key term of theChristological doctrine formulated at the firstecumenical council, held at Nicaea in 325, to affirm that God the Son and God the Father are of the same substance. TheFirst Council of Nicaea, presided over by the emperorConstantine, wasconvened to resolve the controversy within the church over the relationship between the persons of theTrinity. The council condemnedArianism, which taught that Christ was more than human but not fully divine. The use ofhomoousios (Greek: “of one substance”) in thecreed 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 EmperorTheodosius I summoned the second ecumenical council, theFirst Council of Constantinople, which developed and affirmed the earlier creed. The resultingNicene Creed also contained the wordhomoousios and became thedefinitive statement of orthodox belief.