For the purposes of this post at least, I am going to argue that the traditional doctrine of the Trinity is not necessary to Christian faith and practice. It is far from the only logical conclusion one might draw from a reading of the Old and New Testaments. As an aside, I want to say that most Trinitarian theology is hardly Trinitarian at all. We have a bit about God, a tremendous amount about Jesus, and then the appendage of the Holy Spirit which has a kind of vague function.
Now, the Trinity has been necessary for hundreds of years as an article of faith in order to avoid being excommunicated, hunted down, burned at the stake, and so on. Of course, this was not always the case, but has been the case for the vast majority of Christian history. Very very early, the formula of Father Son and Holy Spirit seems to have been adopted. It's universality had to be enforced with violence on a regular basis, but still: what has Christianity not enforced with violence on a regular basis when it comes to doctrine?
The Holy Spirit and the Trinity in the Early Creeds
The Apostle's Creed does no more than mention the existence of the Holy Spirit as part of it's trinitarian structure. It says basically nothing about it. It is implied by the Creed that the HS has something to do with the holy catholic church, the communion of saints, etc., but there is nothing definitive said about the Holy Spirit whatsoever.
At this point, we do not have the modern doctrine of the Trinity. We just have God in three main forms, the third of which is almost completely undefined.
Next is the Nicene Creed, which has more to say about the Holy Spirit. We hear that the Holy Spirit is the Lord and Giver of Life, that the HS proceeds from the Father [and the son], and with the Father and the Son is to be worshipped and glorified. Oh, also, the Holy Spirit spoke through the prophets.
This is still pretty vague, but with the name Lord we get the sense that the HS is a person of some kind, a character in this story at least. Lord of Life, Giver of Life, and speaking through the prophets gives us a vague sense of what this person might be doing. If we glance back, we also find that the Holy Spirit was involved in Jesus' incarnation in the virgin Mary.
Jump ahead to the Athanasian Creed, and suddenly we have 25 lines defining the relationship between the persons of the Trinity. This is followed by the remainder of the Creed, which is dedicated to the incarnation, suffering and resurrection of Jesus. Even in the Athanasian Creed, we can see a difference between the three persons of the Trinity. God is unbegotten. Jesus is not made but begotten of God. The Holy Spirit proceeds from both God and Jesus.
Now, in the Jesus part, we get a little bit about God, since it is important in the Creed to determine exactly what Jesus' incarnation was like. Once that is nailed down, however, we have death, resurrection and judgement. Bang. Done.
Once again, we have nothing whatsoever on the activity, or person-hood of the Holy Spirit. We know more than we'll ever need to know about the Holy Spirit's supposed metaphysics and essence, but nothing at all about the Holy Spirit's character, or story, or activity, or importance.
The point of the Athanasian Creed is clear: quibbling about the metaphysics of the Trinity is a matter of life-and-death. What the third person of the Trinity actually is and does is of no importance whatsoever. Unlike the 25 lines of what I inelegantly call quibbling, it is not saving knowledge. You could theoretically memorize the Creed, ape it on a regular basis, and no one would be the wiser to the fact that you thought the Holy Spirit was in fact a magical canteloupe, or a literal ghost, or that the Holy Spirit was primarily concerned with listening to you argue about homo-ousias. At least as far as this Creed goes.
The Holy Spirit and the Trinity in the Bible
There is sparse evidence pointing to the Trinity in the Bible, and there is no discussion of it as such anywhere to be found. A "full" Trinitarian theology is never articulated in the Bible.
Certainly, the OT talks about the spirit of God, and how that spirit acts as God acts in some circumstances. It doesn't seem clear, to me at least, however that the spirit of God is a different person from God. The spirit of God seems to be what I called in my statement of faith "the manifest presence of God" in the world.
The NT has Jesus describing the Holy Spirit with the person-term Paraclete, clearly referring to the Holy Spirit as a person in that case, rather than exclusively an impersonal force. In other parts of the NT, such as at Pentecost, the Holy Spirit appears as an energy manifesting itself as flame, wind, insight, etc. At Jesus' baptism, the Holy Spirit descends on Jesus as an impersonal force manifesting as something like a dove, and we have God's voice speaking in the same moment.
We also have the baptismal formula of "Father, Son and Holy Spirit." Clearly, the three terms are important, but we have nothing whatsoever of the dense language of the Creeds, which is an invention after-the-fact, dealing with concerns that do not seem to have been the concerns of either Jesus or the first Christians.
We have enough language to talk about the Father, Son and Holy Spirit, but there doesn't seem to be much justification for making any kind of Athanasian formulation a necessary belief.
The Holy Spirit and just plain Holy Spirit
In the NT, what is translated as "the Holy Spirit" is sometimes literally "the Holy Spirit", that is, using the definite article. In other situations, it is simply "Holy Spirit" - no definite article, in a construction that seems similar to "lively spirit" in English. This happens 52 times in the NT. Basically, "pneuma hagion" and "to pneuma to hagion" are translated in exactly in the same way, as "the Holy Spirit", even though in 52 cases this seems incorrect.
What the Greek might imply, though, is that Holy Spirit refers both to a kind of person or divine personality or divine agent, as well as to an experience or, yes, an energy.
What is consistent is that the Giver is consistently "the Holy Spirit" and the Gift is "Holy Spirit". What is implied is that Holy Spirit is more complicated than simply a person. No one would say "the Christ gives the gift of Christ", but we might say "the Holy Spirit gives the gift of Holy Spirit."
-->Actually, I like the idea that "the Christ gives the gift of Christhood", but that's a whole other conversation.
The Athanasian doctrine of the Trinity does not account for this difference in my view.
A bit on Jesus
The Holy Spirit is my entry-point for talking about the trinity because I think there is plenty of justification for using God-language to describe Jesus. There is less justification, however, for using Athanasian language. There are instances in the Gospels when Jesus talks about God being greater; about God being his Father as well as his disciples' Father. There are other instances where Jesus uses the same language of himself and of God. The situation seems to be less clearly symmetrical than the Athanasian Creed describes.
Conclusion for now
This is all a work-in-progress in my own thinking, but at the moment I think that what bothers me most about the Trinity is the symmetry. It seems like a complicated, lived-in, contextual reality in scripture is replaced with a sanitized philosophical structure.
Instead of the Trinity, why not just go with bare description of what we read? God is the creator. Jesus is God's only begotten son as well as God's anointed one. The Holy Spirit is a divine presence as well as an experience or an energy which comes from God and from Jesus as well.
This leaves us open for more reflection as well on our experiences of God. We can talk about Wisdom as she appears - present with God from before the beginning of creation. We can talk about Wisdom, as an example, without worrying whether the doctrine-police are going to get us in trouble for horning in on the Trinity's territory.
We can also, as has been brought up in the comment thread on Aric's post on the Trinity, perhaps take more seriously Jesus's call to be like him. We don't have to be concerned with the problems inherent in trying to be like "the Godhead" (which I've always hated as a term). We can just follow Jesus in being Jesus-like, rather than putting faith in Jesus apart from the belief that we are called to be like him.
I'm still thinking about all of this, and this is more of a mess than I'd like, but there you have it.