Okay ... here's an explanation from Karl Keating.
Filioque
The Western Church commonly uses a version of the Nicene creed which has the Latin word
filioque ("and the Son") added after the declaration that the Holy Spirit proceeds from the Father. Scripture reveals that the Spirit proceeds from the Father and the Son. The external relationships of the persons of the Trinity mirror their internal relationships. Just as the Father externally sent the Son into the world in time, the Son internally proceeds from the Father in the Trinity. Just as the Spirit is externally sent into the world by the Son as well as the Father (John 15:26, Acts 2:33), he internally proceeds from both Father and Son in the Trinity. This is why the Spirit is referred to as the Spirit of the Son (Gal. 4:6) and not just the Spirit of the Father (Matt. 10:20).
The quotations below show that the early Church Fathers, both Latin and Greek, recognized the same thing, saying that the Spirit proceeds "from the Father and the Son" or "from the Father through the Son."
These expressions mean the same thing because everything the Son has is from the Father. The proceeding of the Spirit from the Son is something the Son himself received from the Father. The procession of the Spirit is therefore ultimately rooted in the Father but goes
through the Son. However, some Eastern Orthodox insist that to equate "through the Son" with "from the Son" is a departure from the true faith.
The expression "from the Father
through the Son" is accepted by many Eastern Orthodox. This, in fact, led to a reunion of the Eastern Orthodox with the Catholic Church in 1439 at the Council of Florence: "The Greek prelates believed that every saint, precisely as a saint, was inspired by the Holy Spirit and therefore could not err in faith. If they expressed themselves differently, their meanings must substantially agree. . . . Once the Greeks accepted that the Latin Fathers had really written
Filioque (they could not understand Latin), the issue was settled (May 29). The Greek Fathers necessarily meant the same; the faiths of the two churches were identical; union was not only possible but obligatory (June 3); and on June 8 the Latin
credula [statements of belief] on the procession [of the Spirit] was accepted by the Greek synod" (
New Catholic Encyclopedia, 5:972–3).
Unfortunately, the union did not last. In the 1450s (just decades before the Protestant Reformation), the Eastern Orthodox left the Church again under pressure from the Muslims, who had just conquered them and who insisted they renounce their union with the Western Church (lest Western Christians come to their aid militarily).
However, union is still possible on the
filioque issue through the recognition that the formulas "and the Son" and "through the Son" mean the same thing. Thus the
Catechism of the Catholic Church states that "This legitimate complementarity [of expressions], provided it does not become rigid, does not affect the identity of faith in the reality of the same mystery confessed" (CCC 248).
Today many Eastern Orthodox bishops are putting aside old prejudices and again acknowledging that there need be no separation between the two communions on this issue. Eastern Orthodox Bishop Kallistos Ware (formerly Timothy Ware), who once adamantly opposed the
filioque doctrine, states: "The
filioque controversy which has separated us for so many centuries is more than a mere technicality, but it is not insoluble. Qualifying the firm position taken when I wrote [my book]
The Orthodox Church twenty years ago, I now believe, after further study, that the problem is more in the area of semantics and different emphases than in any basic doctrinal differences" (
Diakonia, quoted from Elias Zoghby’s
A Voice from the Byzantine East, 43).
Tertullian
"I believe that the Spirit proceeds not otherwise than from the Father through the Son" (Against Praxeas 4:1 [A.D. 216]).
Origen
"We believe, however, that there are three persons: the Father and the Son and the Holy Spirit; and we believe none to be unbegotten except the Father. We admit, as more pious and true, that all things were produced through the Word, and that the Holy Spirit is the most excellent and the first in order of all that was produced by the Father through Christ" (Commentaries on John 2:6 [A.D. 229]).
Maximus the Confessor
"By nature the Holy Spirit in his being takes substantially his origin from the Father through the Son who is begotten (Questions to Thalassium 63 [A.D. 254]).