Et in Spiritum Sanctum, Dominum et vivificantem, qui ex Patre Filioque procedit.
And I believe in the Holy Ghost the Lord, and Giver of Life, who proceedeth from the Father and the Son;
From the Nicene Creed, In Latin and the Book of Common Prayer translation
The Latin word Filioque was added to the description of the origin of the Holy Ghost in the Nicene Creed by theologians of the Latin speaking, Rome-based, Western division of the pre-Great-Schism Catholic Church - which is now the Roman Catholic Church.
The Filioque is regarded as one of the major causes of the Great Schism (happening gradually around the year 1000AD) between Western Catholicism, based in Rome and led by the Pope who appointed Archbishops of all nations; and Eastern Catholicism, which was then based in Constantinople and led (largely independently) by the Patriarchs of each nation - with that of Constantinople being senior.
These divisions now continue as the Western Roman Catholic, and Eastern Orthodox* divisions of Catholicism.
[*Eastern Orthodoxy having also a large, administratively separate, subdivision of the Oriental Orthodox (e.g. Coptic, Syrian, Ethiopian etc) - which broke off much earlier in relation to the Monophysite controversy.]
The reason why adding the Filioque was such a decisively divisive issue has several aspects; some of which may not be understandable to modern Men.
Perhaps most fundamental was that the Eastern church believed that authoritative church decisions ought to be permanent; and therefore the Nicene Creed should never be changed. While the Western church believed that the truth was something that emerged over time - and that, for example, theology might discover incompleteness or errors in liturgy, creeds, scripture etc. that ought to be corrected.
But there also seems to have been a difference between West and East over what was the true nature of the relationship within the Holy Trinity; in particular, between the Holy Ghost on the one hand, and the Father and Son on the other hand.
And this difference - in turn - reflected upon the role of the individual Man in Christian life.
The original Nicene understanding and formulation was that the Holy Ghost derived only from the Father - and therefore had no reference to Jesus Christ. This fits with the idea that the Christian life was a thing of God, not of Man, and that individual Men had little role in determining the Christian life, but ought primarily to obey the church.
But it was more than 'obedience'. Orthodox Christianity (with its Father-derived Holy Ghost) is more communal, less individual. Orthodoxy has very little official role for any specific Man - whether by theological study, personal revelation, or through creativity - to make any fundamental or significant effect on the church as a whole.
The guidance of the Holy Ghost is perhaps envisaged as upon the church as-a-whole; not on individual Men.
With the addition of the Filioque; the Western church can be seen as creating a larger and essential role for individual Men - because the Holy Ghost now derived from the incarnated Man Jesus Christ, as well as from the immaterial Father.
This could be seen in terms of a balance between individual Men and the Church; such that both were required for the Christian life.
In general terms - the church was seen as an harmonious unity of individuals; but individual Men could and sometimes should have a decisive effect on the church as-a-whole.
Perhaps this is most obvious in the person of that individual Man who is the Pope of Rome; who has had a special place in the church, as having personal (albeit conceptualized as divinely inspired) authority and capacity to make significant changes - that are regarded as clarifications of original and eternal truth.
But many other individual Men, such as Thomas Aquinas, most notably, have made decisive contributions and wrought church-wide changes.
The Protestants have continued this trend, with a greater role for the individual (e.g. Martin Luther) - but rooted in the same conception of the Holy Ghost deriving from both Father and Son - divinity and Man.
My own understanding of the Holy Ghost can, from this narrative, be seen as continuing this historical trend, by advocating a pure Filioque. That is, I believe that the Holy Ghost is not only derived-from Jesus Christ - but actually is the ascended Jesus Christ.
This seems to me quite clearly stated in the Fourth Gospel (usually called the Book of John) - which I regarded as by far the primary and most authoritative source we have on Jesus's teachings and life.
Also in-line with this narrative direction is that I believe that individual men are now primary in the Christian life; and not "the church" (not any church).
In other words; primary authority and discernment lies in the hearts of individual Men, each for himself; and the churches role is now secondary, and inessential.
Thus the direction of change through the past 2000 years would be (something like) at first regarding the church as primary and essential and Man's duty to obey (proceeding from the Father); then moving-through the intermediate stage when both church and Man are required (proceeding from the Father and the Son); and arriving at my current understanding that the Holy Ghost 'proceeds from', or rather actually-is, the spiritual manifestation-of, The Son, Jesus Christ - a Man*.
*Note: I suppose I should perhaps add, for anyone unfamiliar with this blog; that I regard Jesus as fully-divine - as well as a Man.