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The work of Nicaea was taken up by the second Ecumenical Council, held at Constantinople in 381. This council expanded and adapted the Nicene Creed, developing in particular the teaching upon the Holy Spirit, whom it affirmed to be God even as the Father and Son are God: 'who proceeds from the Father, who with the Father and the Son together is worshipped and together glorified'. The council also altered the provisions of the Sixth Canon of Nicaea. The position of Constantinople, now the capital of the Empire, could no longer be ignored, and it was assigned the second place, after Rome and above Alexandria. 'The Bishop of Constantinople shall have the prerogatives of honour after the Bishop of Rome, because Constantinople is New Rome' (Canon III).
Behind the definitions of the councils lay the work of theologians, who gave precision to the words which the councils employed. It was the supreme achievement of St Athanasius of Alexandria to draw out the full implications of the key word in the Nicene Creed: homoousios, one in essence or substance, consubstantial. Complementary to his work was that of the three Cappadocian Fathers, Saints Gregory of Nazianzus, known in the Orthodox Church as Gregory the Theologian (?329-?90 Basil the Great (?330-79), and his younger brother Gregory of Nyssa (died 394). While Athanasius emphasized the unity of God- Father and Son are one in essence (ousia) the Cappadocians stressed God's threeness: Father, Son, and Holy Spirit are three persons (hypostasis). Preserving a delicate balance between the threeness and the oneness in God, they gave full meaning to the classic summary of Trinitarian doctrine, three persons in one essence. Never before or since has the Church possessed four theologians of such stature within a single generation.
After 381 Arianism quickly ceased to be a living issue, except in certain parts of western Europe. The controversial aspect of the council's work lay in its third Canon, which was resented alike by Rome and by Alexandria. Old Rome wondered where the claims of New Rome would end: might not Constantinople before long claim first place? Rome chose therefore to ignore the offending Canon, and not until the Lateran Council (1215) did the Pope formally recognize Constantinople's claim to second place. (Constantinople was at that time in the hands of the Crusaders and under the rule of a Latin Patriarch.) But the Canon was equally a challenge to Alexandria, which hitherto had occupied the first place in the east. The next seventy years witnessed a sharp conflict between Constantinople and Alexandria, in which for a time the victory went to the latter. The first major Alexandrian success was at the Synod of the Oak, when Theophilus of Alexandria secured the deposition and exile of the Bishop of Constantinople, St John Chrysostom, 'John of the Golden Mouth' (?334-407). A fluent and eloquent preacher- his sermons must often have lasted for an hour or more - John expressed in popular form the theological ideas put forward by Athanasius and the Cappadocians. A man of strict and austere life, he was inspired by a deep compassion for the poor and by a burning zeal for social righteousness. Of all the Fathers he is perhaps the best loved in the Orthodox Church, and the one whose works are most widely read.