"Non-Chalcedonians and Lutherans, members of the Church of the East and Roman Catholics, Calvinists, Anglicans, and Orthodox: all alike worship One God in Three Persons and confess Christ as Incarnate Son of God."
"Yet there is one point in the doctrine of God the Trinity over which east and west seem to part company -- the Filioque....
"The distinctive characteristic of the first person of the Trinity is Fatherhood: He is unbegotten, having His source and origin solely in Himself and not in any other person. The distinctive characteristic of the second person is Sonship: although equal to the Father and coeternal with Him, He is not unbegotten or sourceless, but has His source and origin in the Father, from whom He is begotten or born from all eternity -- 'before all ages' as the Creed says. The distinctive characteristic of the third person is Procession: like the Son, He has His source and origin in the Father; but His relationship to the Father is different from that of the Son, since He is not begotten but from all eternity He proceeds from the Father."
"It is precisely at this point that the western view of the Trinity seems to conflict with that of the east. According to Roman Catholic theology -- as expressed, for example, by St. Augustine of Hippo (360-430) or by the Council of Florence (1438-9) -- the Holy Spirit proceeds eternally from the Father and the Son ("Filioque" ). This doctrine is known as the 'Double Procession' of the Spirit. Now the Greek Fathers are willing on occasion to affirm that the Spirit proceeds from the Father through the Son -- such language is found particularly in St. Gregory of Nyssa -- or that He proceeds from the Father and rests upon the Son; but the Christian east has almost always refused to say that the Spirit proceeds from the Son."
Timothy Ware, The Orthodox Church, Penguin Press (1993).
(Author's Note: Although I have no intention of minimizing the importance of this ancient controversy, I note that the abstract terms -- unity, diversity, and equality -- used here to characterize the Trinity relationship, are all affirmed by both eastern and western Christianity. These same three terms are also found to be sufficiently robust to apply to many other areas of thought, as illustrated in these documents. The Filioque question is of a deeper level than these basic structures, or perhaps we could say, of a higher logical dimension. There may be a sense in which both sides could be right).