If theology is understood in its Patristic sense, as reflection on God as Trinity, the theological issue behind this dispute is whether the Son is to be thought of as playing any role in the origin of the Spirit, as a hypostasis or divine person, from the Father, who is the sole ultimate source of the divine Mystery. The Greek tradition, as we have seen, has generally relied on John 15.26 and the formulation of the Creed of 381 to assert that all we know of the Spirits hypostatic origin is that he proceeds from the Father, in a way distinct from, but parallel to, the Sons generation from the Father (e.g., John of Damascus, On the Orthodox Faith 1.8). However, this same tradition acknowledges that the mission of the Spirit in the world also involves the Son, who receives the Spirit into his own humanity at his baptism, breathes the Spirit forth onto the Twelve on the evening of the resurrection, and sends the Spirit in power into the world, through the charismatic preaching of the Apostles, at Pentecost. On the other hand, the Latin tradition since Tertullian has tended to assume that since the order in which the Church normally names the persons in the Trinity places the Spirit after the Son, he is to be thought of as coming forth from the Father through the Son. Augustine, who in several passages himself insists that the Holy Spirit proceeds from the Father, because as God he is not inferior to the Son (De Fide et Symbolo 9.19; Enchiridion 9.3), develops, in other texts, his classic understanding that the Spirit also proceeds from the Son because he is, in the course of sacred history, the Spirit and the gift of both Father and Son (e.g., On the Trinity 4.20.29; Tractate on Gospel of John 99.6-7), the gift that begins in their own eternal exchange of love (On the Trinity 15.17.29). In Augustines view, this involvement of the Son in the Spirits procession is not understood to contradict the Fathers role as the single ultimate source of both Son and Spirit, but is itself given by the Father in generating the Son: the Holy Spirit, in turn, has this from the Father himself, that he should also proceed from the Son, just as he proceeds from the Father (Tractate on Gospel of John 99.8).Much of the difference between the early Latin and Greek traditions on this point is clearly due to the subtle difference of the Latin procedere from the Greek ekporeuesthai: as we have observed, the Spirits coming forth is designated in a more general sense by the Latin term, without the connotation of ultimate origin hinted at by the Greek. The Spirits procession from the Son, however, is conceived of in Latin theology as a somewhat different relationship from his procession from the Father, even when as in the explanations of Anselm and Thomas Aquinas the relationship of Father and Son to the Holy Spirit is spoken of as constituting a single principle of the Spirits origin: even in breathing forth the Spirit together, according to these later Latin theologians, the Father retains priority, giving the Son all that he has and making possible all that he does.
Greek theologians, too, have often struggled to find ways of expressing a sense that the Son, who sends forth the Spirit in time, also plays a mediating role of some kind in the Spirits eternal being and activity. Gregory of Nyssa, for instance, explains that we can only distinguish the hypostases within the Mystery of God by believing that one is the cause, the other is from the cause; and in that which is from the cause, we recognize yet another distinction: one is immediately from the first one, the other is through him who is immediately from the first one. It is characteristic of the mediation (mesiteia) of the Son in the origin of the Spirit, he adds, that it both preserves his own unique role as Son and allows the Spirit to have a natural relationship to the Father. (To Ablabius: GNO III/1, 56.3-10) In the thirteenth century, the Council of Blachernae (1285), under the leadership of Constantinopolitan Patriarch Gregory II, took further steps to interpret Patristic texts that speak of the Spirits being through the Son in a sense consistent with the Orthodox tradition. The Council proposed in its Tomos that although Christian faith must maintain that the Holy Spirit receives his existence and hypostatic identity solely from the Father, who is the single cause of the divine Being, he shines from and is manifested eternally through the Son, in the way that light shines forth and is manifest through the intermediary of the suns rays. (trans. A. Papadakis, Crisis in Byzantium [St. Vladimirs, 1996] 219) In the following century, Gregory Palamas proposed a similar interpretation of this relationship in a number of his works; in his Confession of 1351, for instance, he asserts that the Holy Spirit has the Father as foundation, source, and cause, but reposes in the Son and is sent that is, manifested through the Son. (ibid. 194) In terms of the transcendent divine energy, although not in terms of substance or hypostatic being, the Spirit pours itself out from the Father through the Son, and, if you like, from the Son over all those worthy of it, a communication which may even be broadly called procession (ekporeusis) (Apodeictic Treatise 1: trans. J. Meyendorff, A Study of Gregory Palamas [St. Vladimirs, 1974] 231-232).