At every Sunday Mass, as we are passing from the Liturgy of the Word into the Liturgy of the Eucharist, we recite the Creed. This is a very ancient Creed, going all the way back to the First Council of Nicaea in 325, where bishops from all over the Empire gathered to define and defend the true Deity of Jesus Christ. It is, to this day, an unparalleled symbol of the Christian faith; it is also a testament to both the essential bonds that hold together the Roman Catholic and the Eastern Orthodox Churches, and to the fact -- and some of the causes -- of our division.
For there is a phrase which is recited by Catholics but not Orthodox. This phrase is "and the Son," Filioque in Latin, and is a reference to a particular belief held by Catholic Christians about the Holy Spirit. (Protestants, since their theological roots are essentially Catholic even when they react against certain elements of the tradition, have retained this.)
All Christians confess one triune God, the Father, the Son, and the Holy Spirit; all confess that the Father begets the Son eternally; all confess that the Holy Spirit proceeds from the Father. In other words, there is one kind of relationship between the Father and the Son, and a differing, an asymmetrical relationship between the Father and the Spirit. The Son is not related to the Father the same way the Spirit is related to the Father -- hence the difference of terminology. But the West, in the tradition of St. Augustine (I do not know whether this is explicit in any earlier theologians), also confesses that the Holy Spirit proceeds from the Son: that is part of the asymmetry. "Through the gift of the Father," as the great doctor has it, the Holy Spirit proceeds not only from the Father but also from the Son; it is this which Catholics say in the Creed and Orthodox do not: Et in Spiritum Sanctum ... qui ex Patre Filioque procedit, "And in the Holy Spirit ... Who proceeds from the Father and the Son."
Why does this matter? The first and most appropriate reply is that what we think about God always matters. But it matters equally because, like it or not, it is an occasion of dispute between Christians; and, while we may be indifferent when we have the luxury of not being directly affected by it, that reality impinges upon us spiritually. Divisions in the Church -- regardless of whose fault they may be, or even if there is no question of anybody being at fault -- are hateful to God and an impoverishment of the holy Church.
The innovation of reciting this in the Creed was introduced in the early Dark Ages, locally in Spain and Gaul, particularly in the Carolingian period (roughly 750-850). This was in response to the Arian and Semi-Arian heretics, who were a substantial element of the Gothic population that had overwhelmed the Western Empire, and who had carried on in spite of their condemnation at multiple councils. It was felt that stating explicitly the belief in the Double Procession of the Holy Spirit, a belief already widely held, would help to reinforce the fact that God the Son was as thoroughly Divine as the Father, as against the Arians who wished to demote the former to the status of an incomparably exalted, but created, being. The Papacy long resisted the addition without disputing the theology, but early in the eleventh century it took place at Rome itself, and was the occasion of a deeper breach between the Latins and the Greeks. Eventually, in 1054, a Papal legate in Constantinople, losing patience in his negotiations with the Patriarch of that city, declared him excommunicated and deposed; and, gradually, the schism coalesced, and has never been decisively healed.
What, then, are the arguments for and against?
I cannot speak with total authority: my understanding of Orthodox theology to the extent that it differs from that of Catholics is very imperfect. However, to my understanding, the objections generally tabled by our brethren in the East are as follows:
1. That no additions should have been made to the Creed without the summoning of an Ecumenical Council -- the authority of the Pope was not sufficient to make such an addition;
2. That the Bible does not teach that the Holy Spirit proceeds from the Son;
3. That to assert the Filioque blurs the distinction between the Father and the Son, or even that it amounts to a blending of all three Persons, thus destroying the integrity of the Trinity.
1 we shall have to leave aside. This is not because there is no weight to the objection, but because, strictly speaking, it is not theologically relevant. The question of what exactly the primacy of the Holy See means is an important, indeed an indispensable, theological question; but a different one.
2 is technically true, but I must admit I regard it as a very weak argument. The fact that something is not explicitly present in Scripture settles very little -- for that matter, the Arians could make out a persuasive case that the very Deity of the Son was not Biblical. Any assertion made a silentio is on extremely shaky ground from a logical perspective.
3 has always struck me as slightly odd. For the formulation of this objection with which I am acquainted is that, as the Orthodox convert Timothy Ware puts it, "If the Son as well as the Father is an arche, a principle or source of the Godhead, are there then ... two separate principles in the Trinity? ... Orthodox theology upholds the 'monarchy' of the Father within the Trinity: he alone is the arche, the source or origin of being within the Godhead. But western theology ascribes this distinctive characteristic of the Father to the Son as well, thus fusing the two persons into one ..." (The Orthodox Church, pp. 213-214)
Now, the oddness of this objection is that it totally fails to take into account the transparent fact that the Son has His origin (for lack of a better word) in the Father. To say that the Spirit proceeds from the Son does not imply that the Son is independent of the Father; it merely adds that the relations between the Spirit and the other two Persons are symmetrical rather than asymmetrical. And when we consider that the Catholic Church has always maintained that the Spirit's procession from the Son is a gift from the Father to the Son, this objection seems to have lost its teeth entirely.
If that seems too abstract, think of three men in a family, A, B, and C. A is the father of B, and B is the father of C; A is therefore the grandfather of C, and is thus C's "point of origin" as well as B's. Now, is it any threat to A's stance at the head of this familial line to point out that B, in his own capacity, is a father as well? Does it lessen the distinguishing characteristics of A somehow, or blur the distinction between A and B? I should say not.
But what of positive arguments? After all -- and even assuming that these replies satisfy Orthodox objections, which I naturally cannot answer, although they satisfied me -- merely to disprove arguments against the Filioque does not constitute proof. But I think there is evidence for it, Biblical evidence, and evidence set forth explicitly in the uncontested elements of the Nicene Creed.
For consider. The substantial oneness between the Father and the Son -- that is what the Creed was written to defend. And it received its theological base precisely from such passages as this:
He is the image of the invisible God, the firstborn of all creation. For by him all things were created, in heaven and on earth, visible and invisible, whether thrones or dominions or rulers or authorities—all things were created through him and for him. And he is before all things, and in him all things hold together. And he is the head of the body, the church. He is the beginning, the firstborn from the dead, that in everything he might be preeminent. For in him all the fullness of God was pleased to dwell, and through him to reconcile to himself all things, whether on earth or in heaven, making peace by the blood of his cross. (Colossians 1.15-20)
Now, insofar as the Son is the image of the invisible God, is it inappropriate to suppose that He is a perfect image -- that, "by the gift of the Father" as St. Augustine put it, every feature of the Father is reproduced in the Son, up to and including the origination of the Spirit? That position would, of course, be stronger if Christ were called the exact image of the Father's being.
Long ago, at many times and in many ways, God spoke to our fathers by the prophets, but in these last days he has spoken to us by his Son, whom he appointed the heir of all things, through whom also he created the world. He is the radiance of the glory of God and the exact imprint of his nature, and he upholds the universe by the word of his power. (Hebrews 1.1-3a)
I honestly cannot see a way of construing this that makes the Filioque anything but overwhelmingly probable. That is not, of course, conclusive; but I hope it can serve to advance the discussion.