I am convinced, well sort of, all right, I am not totally sure, but you have to start with a bold statement to get attention, so here it goes. If it wasn’t for Jesus’ teaching on the Holy Spirit in John 14-16 and Paul’s in 1 Corinthians 12 – 14, I don’t think we would have the doctrine of the Trinity. It would be the doctrine of the Two-ity (isn’t there a cooler way to say that?). Before you throw me under the bus, hear me out. The Old Testament presents the Holy Spirit in a similar way to how it presents a person and his spirit. What I mean is that the Spirit of the OT doesn’t seem to be a separate entity, as much as a way to describe how God does things (He pours out His Spirit, He sends His Spirit to…). In the New Testament, many passages could be read in this exact same way. Sure, you can look at them with the benefit of John and Corinthians and say, “this totally points to the Trinity and the Spirit being separate.”
At this point, there is a smartypants out there who would say, “What about the book of Acts? huh?”. It is true, that Acts belongs in the list with John and 1 Corinthians in being the best source of teaching the Bible gives on the Holy Spirit. However, since it is narrative, I think you could still make the argument that it is merely talking about God’s Spirit at work, and not a separate “person” in the Godhead. It is this lack of teaching on the Holy Spirit that can lead many to wonder whether the Holy Spirit should be discussed as part of the “Trinity”. I can still remember speaking with a fellow Bible school staff about 6 years ago about his struggles with the Spirit in the Trinity. It was like he was confessing some deep, dark secret or something. I think he felt hesitant to voice he feelings that he was doubting the Trinity in favor of the Two-ity. Are there any other closet Holy Spirit Trinity doubters out there right now? Another reason so many struggle with this is that the “separateness” of Jesus and God seems much clearer due to the incarnation. You have one in the flesh, and the other maintaining “spirit” only form. We can connect with Jesus easier and see the distinction. Add to this the lack of passages with direct teaching as I mentioned before, and we have the recipe for doubt.
Consider all the passages in the Old Testament that connect the Spirit to God in a possessive fashion. From the very beginning, Genesis 1:2 states, “the Spirit OF God was hovering over the waters.” David’s famous plea in Psalm 51:11 says, “take not YOUR Holy Spirit from me”. David doesn’t say, “take not THE Holy Spirit from me.” Isaiah 63:10 says, “But they rebelled and grieved HIS Holy Spirit” not “the Holy Spirit”. This possessive phrase can be found in at least 14 more Old Testament passages, all using “of” and not “the”. There are a few passages in the Old Testament like Ezekiel 8:3, “the Spirit lifted me between earth and heaven.” However, just a few chapters later in 11:24, he uses “Spirit of God” (he also uses “My Spirit” 3 times as well). In fact, finding a “the Spirit” in the Old Testament without “the Lord’s”, “of God”, “Your”, or “His” are few and far between. This is why I said earlier that with only the Old Testament, we don’t see a firm separation of “The Holy Spirit”, but rather would view it as “God’s Spirit”.
We already discussed the strong monotheism of the Jews in Old Testament times. No serious rabbinic literature teaches the Holy Spirit as a “separate person” from God. Deuteronomy 6, “The Lord is one” rules out to them any possibility of viewing the Spirit in the Old Testament as “The Spirit”. Even after Jesus’ teaching, I wonder if many of the disciples would have agreed with assigning a separation of the Spirit. They were having a hard enough time understanding the relationship between Jesus and God! It’s a good thing that we do have the New Testament, and next time, we will piece together whether it fully affirms the “personhood” of the Holy Spirit. Until then, if you are doubting the Trinity doctrine, just keep it to yourself. You never know when your friend will start a Bible blog and use you as the example. Just kidding! Sort of…. maybe…
The Bible Nerd 
Despite Acts being a narrative I think you can still deduce no small amount of info on the Spirit’s divinity. “Then Peter said, “Ananias, how is it that Satan has so filled your heart that you have lied to the Holy Spirit and have kept for yourself some of the money you received for the land? Didn’t it belong to you before it was sold? And after it was sold, wasn’t the money at your disposal? What made you think of doing such a thing? You have not lied just to human beings but to God.” – (Acts 5:3-4)
As for me, Im no closet doubter of the Trinity, however I do not see when people list the “Requirements” of salvation and they include belief in the Trinity as being one of those cardinal beliefs damnable if not received (and can be an actual biblical anathema). Indeed we do need to know who God is, and not believing in the Tri would not be knowing who God is, but there are many aspects that Im sure I dont know about God, so why is this one lack of understanding so damnable? I mean it took while, at least from what i see in the fathers, for really a full on teaching to come out about the union – yes it was kind of believed in embryo, that is to say, there seemed to be some sort of holy mystery there and some sort of relation between the 3 (namely the 2 as you pointed out) prior to the councils and apologetic writings, but it was never a real clear message, it is one that has to be deduced as and compiled.
People always say that Jesus HAD to be incarnate for the atonement to be effectual, but I dont see it (I do see the need for sinlessness though). I know your not saying anything like this at all, and I know I sound a little heretical (let me remind you I do believe in the Tri), but I just dont see why it is a fundamental belief needed for glory since it actually is a lil vague.
That said, I already posted a comment, and I dont want to bogart your comment sections, so no-need to respond (perhaps you can incorporate in part of a future post). Thanks for the post and making me think!
Jason, I would agree with you that having doubts about the doctrine of the Trinity, or questioning how parts of it are worded, doesn’t mean you just lost your salvation. I do think that there is dangerous ground here though, as the Jehovah’s Witnesses and Mormons have shown. For these groups, they ended up not only questioning the Trinity, but also denying the full divinity of Jesus. Not believing that Jesus is God is much more serious than not agreeing with the use of the word “person” for example. Any time you have doctrine which includes an “apparent paradox” or “mystery of the faith” like the Trinity does, I think we need to be careful about drawing strict lines and getting ready to burn people at the stake. You are also in correct in stating that the doctrine that we know of as the Trinity didn’t come into full detail until the church had already been going for hundreds of years. There are small glimpses in the early church fathers, but nothing systematic and no creeds of course until the 300’s AD. How are all these people in heaven without a set agreement to a doctrine that wasn’t even taught when they were presented with the Gospel? Some doctrines are so clear in the Bible that can be listed as “foundational” or a must believe like salvation by faith. The Trinity is not one of these in my view (what I mean is a clear explanation of the three in one that theologians provided later).
I would also agree that Acts is a very rich source of teaching on the Holy Spirit. My point is that it isn’t didactic teaching like the letters or even Jesus’s teaching contained inside the narrative of John. It is embedded in historical narrative, and so needs the direct teaching of the letters to shed full light. For example, in Acts 8, the Samaritans believe Philip and the Gospel message, but haven’t yet “received” the Holy Spirit. The same is said of the Ephesian disciples in Acts 19 (did you “receive” the Holy Spirit when you believed?) I realize this issue is compounded by the English word used to translate the Greek, but still, it is an issue. It would appear that you become a Christian and then later “receive” the Spirit. Thankfully, we use clear didactic teaching in the letters to dispel this notion. In Romans 8, Paul says if you belong to Christ then you have the Spirit. When Paul describes salvation in Titus 3, he says the Holy Spirit cleanses us at the moment of salvation.
That is what I meant about what we learn in Acts about the Holy Spirit at times needs and can be interpreted through the direct teaching in the letters and teaching of Jesus contained in the Gospels.
Great questions! Keep them coming!