My favorite part of Pneumatomachianism is its ability to demonstrate to the contemporary Church how slowly theological development moves. This is demonstrated by the first two versions of the Nicene Creed. The original text, written and agreed to at the Council of Nicaea in 325, simply mentioned "And in the Holy Spirit." That was it. That was the only part of the statement of faith that dealt with the Holy Spirit. By 385, the text had been significantly elaborated to a version much closer to the one used today: "And I believe in the Holy Ghost the Lord, and Giver of Life, who proceedeth from the Father; who with the Father and the Son together is adored and glorified; who spake by the Prophets."
The gap between the two creedal statements shows that the church was initially concerned with the divinity of the Son ("light of light, true God of true God") and that the divinity of the Spirit, while perhaps a logical consequence of the Son's divinity, was (at best) an afterthought. More accurately, it was a "hardly-thought." A positive doctrine of the Trinity wasn't attempted until others (the Pneumatomachians) had declared firmly their denial of the Spirit's consubstantiality. It's as if they said, "Hey, alright, you win on the whole "Son is divine" thing. But surely you can't expect us to say that the Spirit is God too. That's just crazy." And, sure enough, the church wasn't sure...at first.
As the writings of the various Church Fathers who helped develop our doctrine of the Trinity show, it took them a long, long time to figure out that the Holy Spirit is God. Around 360 CE, Athanasius argued for the consubstantiality of the Spirit from the Son-is-divine-so-the-Spirit-must-be-too angle. But he didn't come out and use words like "God" or "Trinity." Basil, the hero of Trinitarian orthodoxy, is another example of reticence. He began from the other side of the debate, at first insisting that the Spirit can't be created. From there, he moved to sharing the divine substance. And then all the way to equal and to be glorified along with Father and Son, but he never called the Spirit God (JND Kelly). Instead, it seems his fear of overstating orthodoxy held him back.
In some ways, the Pneumatomachians' argument is compelling. The Father and Son have just about the only divine relationship that makes sense. Since the Son is the "only-begotten", to add a third person of the Trinity requires a new concept. And if the Spirit can't be "the other Son," what will we call its relationship to the rest of the Trinity? Pet? Sister? Next-door neighbor? You see? The Father-Son thing was good. Why would we want to go and mess it up? Likewise, the bible doesn't have a lot to say about the Spirit's divinity. In fact, in most passages, it appears to be the "gopher" (a term used by a member of the class) or a servant of the Father and Son. And that's hierarchialism, a heresy we won't get to in this series. Either way, the Spirit's divinity seems a bit of a stretch.
So why believe in the Spirit's divinity? And, more basically, why believe in the Trinity at all? My best answer is this: relationality. The Father and Son must be distinct persons of the Trinity in real relationship with each other in order for God to be relational and not static and lifeless. And, if the Father and Son are relational, then their relationship necessitates a third party--the Spirit. The Spirit is the overflowing of their love for each other. The Spirit is the Trinity's perfect internal relationship. Without the Trinity, we get a God who isn't able to love us, save us, or intervene in human affairs in any meaningful way. But how that works? Well, it's a mystery.
I don't like hiding behind things I don't understand, but that's pretty much where I end up. I'm relying on logic and history and tradition and scripture and none of those gives me a definite understanding of why the Trinity. Even the video below, which is good, raises as many questions as it answers. Perhaps you have more to say about it. If so, please, PLEASE let me know.
Here's the slide show. Enjoy. Also, below it is a video from the Orthodox Church on the Trinity. It's mostly good. A few bits (divine vs deity) are the Eastern way of saying things that the Western Church says a little clearer, but, for the most part, it's good.