The Holy Spirit is the Third Estate of the Trinity

There isn't a lot of support in the Xian canon for the Trinity, any mention 'Holy Spirit' is used for Trinitarian enthusiasts.  Romans 5:5, " And hope does not disappoint, because the love of God has been poured out in our hearts through the Holy Spirit, who is given to us", is it "which is given to us"?  The Holy Spirit here and elsewhere seems to mean more the high spirits of the religious feelings rather than a personification who possesses one like a ghost.

I Corinthians 12:13 "We were all baptized into one Spirit to become one body whether Jews or Greeks, whether slaves or free, and we were given one Spirit to drink." This clearly means identification with the esprit de corps, and is not a reference to some sort of being of quasi-being or quark of God.

Galatians 4:6 "Because you are sons, God sent the Spirit of his son into your hearts."  But the canonical Holy Ghost is not a Spirit "of" his son, it's awkward wording for a trinitarian reading, the Spirit is "the spirit o' the son", a feeling not an entity.  

See what I mean?  These are weak props violating the strict sense of the concept of monotheism.

I think the Holy Spirit was deified to enthrone the original enthusiastic element of the early church in what was becoming a more hierarchical organization, to say that their experience in communion was a significant aspect of the whole.

The Arians and other Unitarians did not see the logic behind Trintarianism, because it wasn't logical; it was an emotional projection, a loss of logic to mysteries of faith, and its fun magical wonders.  It is a backwash from the pagan religions and philosophies of the Mediterranean, Egyptian elaborations and Hellenic sophistries, back into the strict monotheism of the Jewish religion.  The Gentiles came from backgrounds that allowed for flexible God grouping.  So the Son of Xian belief grew from being a human son of Mary, an itinerant teacher, to a celestial being every bit as important as old Yahweh and of one substance, and the communion of the faithful was subsequently added part and parcel to this fusion.  This is the politics of the Trinity.  Jesus is the newer Yahweh, not just for Jews, and as proof that Gentile congregations were blessed, the Holy Spirit represented evidence that of presence.

Just writing about this 3-in-1 logic is work.

But the Holy Spirit visits the faithful, palpably for the imaginative faithful, through intoxicating group enthusiasms, and through the wonders of voices in the head and the like.

One more citation.  I Corinthians 12:1-11:
1 Now concerning spiritual gifts, brethren, I do not want you to be uninformed. 2 You know that when you were heathen, you were led astray to dumb idols, however you may have been moved. 3 Therefore I want you to understand that no one speaking by the Spirit of God ever says "Jesus be cursed!" and no one can say "Jesus is Lord" except by the Holy Spirit. 4 Now there are varieties of gifts, but the same Spirit; 5 and there are varieties of service, but the same Lord; 6 and there are varieties of working, but it is the same God who inspires them all in every one.  7 To each is given the manifestation of the Spirit for the common good. 8 To one is given through the Spirit the word of wisdom, and to another the word of knowledge according to the same Spirit, 9 to another faith by the same Spirit, to another gifts of healing by the one Spirit, 10 to another the workings of miracles, to another prophecy, to another the discernment of spirits, to another various kinds of tongues, to another the interpretation of tongues. 11 All these are inspired by one and the same Spirit, who apportions to each one individually as he wills. 

Like what the Iroquois term Orenda.  You may translate that as "The Force" if unacquainted with the beliefs of the older Haudenosaunee, who maintained independence and empire from upstate New York through the Midwest during the period 1620-1781 when the whites were encroaching from over the ocean.