In the Western version of the Nicene Creed (or the Niceno-Constantinopolitan Creed), a change was made in the article on the Holy Spirit. In Greek, it says (translated into English) that the Holy Spirit “proceeds from the Father.” In later Western or Latin editions, which is the basis for most of the Western churches, including (most) Anglican and Lutheran ones, this was altered to say that the Holy Spirit “proceeds from the Father and the Son.” This is known as the Filioque controversy. Some Greeks have said that this means that these traditions teach that the Spirit proceeds from the Father and the Son as from two different principles. That, however, is not the case. In this post, I will try to explain why I think the Filioque is not just acceptable but central.
The interpretation of the Filioque in Western Christianity has always been (and remains) that the Spirit comes from the Father through the Son. Latin Christianity – whether Roman, Anglican, Lutheran, etc. – holds that the Spirit proceeds from one principle; the Father. So does the East. I do not think there are any definite theological differences between the two positions.
The traditional western approach does not state that the Father and the Son are two distinct principles but that the Father is the one principle from whom the Spirit proceeds, but that He does so through the Son. This principle was also held by Tarasius I, who was elected Patriarch of Constantinople in 784. In a profession sent to pope Hadrian I, prior to the seventh ecumenical council (and read at said council), he stated that the Holy Spirit proceeds “from the Father through (Gk. diá) the Son.” He did not change the text of the Niceno-Constantinopolitan Creed but he did acknowledge that when we say that the Spirit proceeds from the Father, this implies that He does so through the Son. And the reason why that is important is that if He did not proceed through the Son, there would be no relation between the Spirit and the Son. And since the relations are what constitutes personhood in the godhead, the Son would either be ontologically subordinate to the Father and the Spirit or we would have to say that the doctrine of the Trinity is polytheistic, not monotheistic.
Some have noted that since Latin has more words for ‘and,’ it allows for more distinctions than Greek. Saying that the Spirit proceeds from the Father kaí the Son (kaí being ‘and’ in Greek) would seem to entail that the Father and the Son were two different principles, which would make the Spirit subordinate to both and his relation to them would be identical, and thus you could not distinguish properly between the persons. In Latin, however, you can use the word et (which is basically kaí) or the word que (as in Filioque) which allows for more distinctions.
The traditional western teaching is that the Spirit does indeed proceed from the Father and the Son, but from one principle, from the Father, through the Son. But that does not mean that you should include the Filioque in the Greek version of the Creed (which the Roman Catholic Church does not do, by the way). And maybe we should not have it in the Western version of the Creed either. But I think that it being there is good, as it helps us understand the relations in the Godhead. The Filioque is important in that it shows us that the Spirit is not subordinate to either the Father or the Son but also that the Son is not subordinate to the Father (or the Holy Spirit).
 See Joannes Metaxas-Mariatos, The Filioque Controversy: Chapters from the eastern orthodox reaction: An historical-theological perspective (Master’s thesis, Durham University, 1988), 8-9.
 I also think that without the Filioque, it would be hard to argue for divine simplicity, but that is a discussion for another day.
 Some Eastern theologians argue that it is improper because it was introduced without conferring with Eastern theologians. I find that argument unpersuasive, for one very important reason. At Nicea, in 325, the Creed had this much to say about the Spirit: “And [we believe] in the Holy Spirit.” This, however was changed at the second ecumenical council in Constantinople in 381: “And [we believe] in the Holy Spirit, the Lord and Giver of life, who proceeds from the Father, who with the Father and the Son together is worshiped and glorified, who spoke by the prophets.” See here. And guess what, there were no Western theologians present at the second ecumenical council.
Edit: I see no principled difference here. If the argument is that it needs to be affirmed to become an ecumenical council, I wonder why the same logic is not applied to the fourth ecumenical council in 451. Large portions of the oriental Church did not approve. Yet the Greeks did not claim that it was not an ecumenical council. The argument, then, boils down to ‘rules for thee but not for me.’ The West, of course, does not approve that. What is good for the goose is good for the gander.