homoousios (Greek, ‘of the same substance’), a concept central to the Christian doctrine of the Trinity, enshrined in the Nicene Creed of A.D. 381. It attests that God the Son (and by extension the Spirit) is of one and the same being or substance (ousia) as the Father. Reflecting the insistence of Athanasius against Arianism that Christ is God’s eternal, coequal Son and not a creature, the Nicene homoousios is also to be differentiated from a rival formula, homoiousios (Greek, ‘of similar substance’), which affirms merely the Son’s likeness in being to God. Though notoriously and superficially an argument over one Greek iota, the issue was philosophically profound and theologically crucial whether or not Jesus of Nazareth incarnated God’s own being, revealed God’s own truth, and mediated God’s own salvation. See also TRINITARIANIS. A.E.L.