By Fr. John Romanides
The Filioque is a heresy, because it confuses the hypostatic properties of the Father, i.e. His being cause, with those of the Son and, as a result, introduces a kind of Semi-Sabellianism. This is the case if the notion of being cause belongs both to the Father's and to the Son's hypostasis, but not to the Spirit's. If the Father and the Son as hypostaseis are the cause of the existence of the Holy Spirit, then, according to Photios, we have two principles in the Godhead, or, if they think of the Father and the Son as one cause, then, as we said above, we have Semi-Sabellianism, i.e. the identification of the incommunicable, hypostatic properties of the Father and the Son. If the cause is identified with the essence and not with the hypostaseis, then the Holy Spirit is a creature, because the doctrine that the essence is the cause of another person is the doctrine of the Eunomians, since they identified the cause of the existence of the Son with the essence of the Father and attempted on this basis to demonstrate that the Son is a creature. Consequently, if the Father's and the Son's essence is the cause of the existence of the Holy Spirit, then the Holy Spirit is a creature. Again, He is a creature, if the cause of the Spirit's existence, or His procession, is a common energy of the Father and the Son, of which the Spirit is lacking. This is the case, because, as Orthodox and Pneumatomachians argue, the lack of even one energy common to the Father and to the Son from the Spirit would demonstrate the created nature of the Spirit. The one doctrine leads to Semi-Sabellianism and the other to Eunomianism, or to the heresy of the Pneumatomachoi where the Spirit becomes a creature.
Today, the Latins are obliged, if they wish to revise the foundation of their theology, not only to take seriously the theology of the Fathers, which constituted the basis of the decisions of the First and Second Ecumenical Councils, but also to revise the Trinitarian terminology, which is based on Augustine's doctrine.
Source: An Outline of Orthodox Patristic Dogmatics, pp. 33-35.