The Spirit says

I'm going to comment on some related passages of Scripture, then consider their implications:

But the Spirit entered into me and set me on my feet, and he spoke with me and said to me, “Go, shut yourself within your house (Ezk 3:24; cf. 2:1-3).

Syntactically, the speaker seems to be the Spirit of God. God's Spirit enters Ezekiel and then proceeds to speak to him. 

However, I suppose it's possible that the speaker is the glory of Yahweh, in the preceding verse. 

Strictly speaking, the "Spirit" in v24 (cf. 2:2) isn't identified as God's Spirit. However, it can't be Ezekiel's own spirit that enters him, since Ezekiel's own spirit is always present in Ezekiel. In context, it can't be an evil spirit. 

No indication of an angelic spirit. Moreover, angelic spirits are never said to enter people, although there's a sense in which an angel appearing to someone in a dream might qualify, but that's not the context.   

And the Spirit of the Lord fell upon me, and he said to me, “Say, Thus says the Lord: So you think, O house of Israel. For I know the things that come into your mind (Ezk 11:5).

This is more explicit. The speaker is the Spirit of God. This is direct speech. The Spirit addressing Ezekiel, like an audible voice. And this verse might help to clarify the referent in Ezk 2:2 & 3:24. 

In principle, the Spirit could inspire a prophet at a subliminal level, so that what the prophet things and says is the result of inspiration, even though he doesn't hear a voice speaking words to him. But that's not what this verse says.

And the Spirit said to Philip, “Go over and join this chariot” (Acts 8:29).

Another example of the Spirit speaking directly to someone.

And while Peter was pondering the vision, the Spirit said to him, “Behold, three men are looking for you (Acts 10:19).

Yet another example of the Spirit speaking directly to someone.

While they were worshiping the Lord and fasting, the Holy Spirit said, “Set apart for me Barnabas and Saul for the work to which I have called them” (Acts 13:2).

This might be direct speech, or it might be shorthand for Christian prophets who convey a revelation to Saul and Barnabas. 

Now the Spirit expressly says that in later times some will depart from the faith by devoting themselves to deceitful spirits and teachings of demons (1 Tim 4:1).

Paul seems to be alluding to a message by Christian prophets. If so, that's indirect speech. The Spirit speaking through a second party. 

Therefore, as the Holy Spirit says, “Today, if you hear his voice (Heb 3:7; cf. 10:15-17).

In context, this is indirect speech. The Spirit as the source of Scripture. 

That involves a dialectical interplay. For a reader, the Spirit's message is mediated by Scripture. But for a prophet, Scripture is mediated by the Spirit. The Spirit's message becomes inscripturated. 

This is a common Biblical theme. The Spirit inspires prophets to speak the words of God. In addition, the Spirit inspires revelatory dreams and visions, which may include auditions. 

He who has an ear, let him hear what the Spirit says to the churches (Rev 2:7).

Here we have two speakers: Jesus and the Spirit. The Spirit inspires John to convey what Jesus says.

Now let's take stock:

1. In some cases the Spirit speaks directly to somebody, in an audible voice. That's something only a personal, external agent can do. Moreover, that requires intelligence.

That's true even if the voice is telepathic. For the prophet hears a voice that's not a figment of his own imagination. It's not like interior monologue, where he wills himself to hear a mental voice. Rather, this is one mind interacting with another mind. Temporary possession. 

And it's not interior monologue as a rhetorical device (e.g. Ps 42). These are prosaic rather than poetic descriptions. Nothing like the genre of Ps 42. 

2. In other cases, the Spirit speaks indirectly by inspiring prophets to speak the words of God. Once again, that requires the Spirit to be a rational agent. To convey a message requires an intelligent messenger. Informing the prophet.

In addition, it requires a divine agent to inspire divine words. The product of the Spirit's agency is the very word of God. 

3. In these passages, is the Spirit just a synonym for God? Could you substitute "God" for "Spirit"? 

That depends on what you mean by "God". If by "God", you mean the Father or the Trinity, then the Spirit isn't God in that respect. But the Spirit is "God" in the sense of fully divine, just like the Father.

If the Spirit is just a synonym for God, why does Scripture so often attribute actions to the Spirit rather than God? That implies some sort of internal duality within the Deity.

4. Does Scripture represent the Spirit as a personal agent due to personification? But what would the Spirit personify? If the Spirit is a personification, then it personifies something impersonal. Yet the actions and properties in view are inherently rational. It's not like ascribing intelligence or intentionality to inanimate objects (e.g. "The moon methinks looks with a watery eye; And when she weeps, weeps every little flower"). 

If the Spirit personifies anything, that has to be the Deity. But it's nonsensical to personify a personal agent. That's not how personification works.

If you say the Spirit personifies the wisdom of God, that's a divine attribute. A personal attribute. That's inseparable from the Deity himself.  

It's not personification like Lady Wisdom and Lady Folly in Prov 8-9, where they function as fictional characters in parabolic allegories. It's not like Cupid, which is clearly imaginary.  

5. Moreover, to say rational ascriptions to the Spirit are just personifications is a double-edged sword. An atheist will take that one step further and say the voice of God is a personification of the prophet's psychotic imagination. A deified hallucination. A self-projection.