Answer: Filioque is Latin for "and the Son." It is found in the Nicene Creed as it is said in
the Catholic Church: "I believe in the Holy Ghost... Who proceeds from the Father and the Son." The controversey surrounding it is one of the ancient dogmatic points of disagreement between the Catholic Church and the schismatic churches of the East.
When he stirred up trouble between Constantinople and Rome in 870, Photius needed an occasion to bring a popular movement against the Latins. This he found when certain Spanish monks chanted the Filioque in the Creed of their Mass. Photius cliamed then, and the schismatic Greeks still claim, that this addition to the Creed was not permissable. Photius' followers held that the Council of Ephesus, in its 7th canon, forbad additions to the creed. ("It is not permitted to produce or write or compose any other creed except the one which was defined by the holy fathers who were gathered together in the Holy Spirit at Nicaea.") This is a false premise, since the canon was written to forbid the composition of any teaching contrary or contradictory to any truth already expressly defined in the Creed of Nicea-Constantinople. Other creeds had been used before and after Nicea, witness the one attributed to St. Athanasius. And witness also that the Creed of the Council of Nicea was itself reformulated by one Ecumenical Council which took place inbetween Nicea and Ephesus: Constantinople I.
As the reader has no doubt garnered by now, the original Creed formed at the Council of Nicea, and later added to at the First Council of Constantinople, did not originally contain the Filioque, which was first added to the Mozarabic Liturgy by the Council of Toledo around the year 600. (The Visigothic Kingdom was a stronghold of Arianism and other Trinitarian heresies, so the Mozarabic bishops, properly exercising their office, inserted the word to defend Trinitarian orthodoxy.) From the Mozarabic Rite it made its way into the Gallican Rite, formally being added to their liturgy at the council of Aachen around 800. In the 11th century, Pope Benedict VIII formally added it to the Roman Rite, which had, by that time, imported much from the Gallican Liturgy.
Now that we have identified the issue and explained a little of the historical controversy surrounding it, it remains for us last to defend the truth of the dogma of the Filioque -- a dogma one denies at the peril of his soul.
Concerning our dogma, Father Anthony J. Maas, the great Catholic Scripture scholar says, "As to
Sacred Scripture, the inspired writers call the Holy Ghost the Spirit of the Son (Gal 4:6), the Spirit of Christ (Rom. 8:9), the Spirit of Jesus Christ (Phil. 1:19), just as they call Him the Spirit of the Father (Matt. 10:20), and the Spirit of God (1 Cor. 2:11). Hence they attribute to the Holy Ghost the same relation to the Son as to the Father. Again, according to Sacred Scripture, the Son sends the Holy Ghost (Luke 24:49; John 15:26, 16:7, 20:22; Acts 2:33; Tit. 3:6), just as the Father sends the Son (Rom. 8:3, etc.), and as the Father sends the Holy Ghost (John 14:26). Now, the 'mission' or 'sending' of one Divine Person by another does not mean merely that the Person said to be sent [only apparently] assumes a particular character [...], as the Sabellians maintained; nor does it imply any inferiority in the Person sent, as the Arians taught; but it denotes, according to the teaching of the weightier theologians and Fathers, the Procession of the Person sent from the Person Who sends. Sacred Scripture never presents the Father as being sent by the Son, nor the Son as being sent by the Holy Ghost. The very idea of the term 'mission' implies the persons sent goes forth for a certain purpose by the power of the sender, a power exerted on the person sent by way or a physical impulse, or of a command, or of prayer, or finally of production; now, Procession, the analogy of production, is the only manner
admissible in God. It follows that the inspired writers present the Holy Ghost as proceeding from the Son, since they present Him as sent by the Son. Finally, St. John (16:13-15) gives the words of Christ: 'What things soever He [the Spirit] shall hear, He shall speak;...He shall receive of mine, and shall shew it to you. All things whatsoever the Father hath, are mine." Here a double consideration is in place. First, the Son has all things that the Father hath, so that He must resemble the Father in being the Principle from Which the Holy Ghost proceeds. Secondly, the Holy Ghost shall receive 'of mine' according to the words of the Son; but Procession is the only conceivable way of receiving which does not imply dependence or inferiority. In other words, the Holy Ghost proceeds from the Son."
Eastern Church Fathers who can be cited in defense of this dogma are Sts. Basil the Great, Gregory Nazianzen, Gregory Thaumaturgus, Gregory of Nyssa, Cyril of Alexandria, and Hippolytus. Nor should we forget that great Eastern profession of Faith, the Athanasian Creed, which proclaims: "The Holy Spirit is of the Father and the Son, not made nor created nor begotten but proceeding."