Acts is about God. Like every other book in this Bible (yes I'm including Leviticus), Acts is about God. Acts is particularly about the work of the Spirit of God, the third person of the Trinity, after Jesus's ascension into heaven. Now I want to point something out. This is not new. The Holy Spirit didn't magically show up in Acts for the first time in history. We've talked about Him throughout the Bible so far, but more specifically, we talked about His presence in the book of Luke (if you didn't know, Luke and Acts were essentially written as a part 1 and part two of the same story, written by Luke). At the same time, this is an amazing fulfillment of the promise of the Holy Spirit, given by Jesus in John 14.
So here is the work of the Spirit in Acts. First, the Holy Spirit is the origin of Scripture (1:16; 3:18; 4:25; 28:25). This isn't a new thing when we read it in 2 Timothy 3:16 or in 2 Peter 1:20-21. Obviously they knew that the writings of the Old Testament writers (Acts mentions Isaiah, David, and "all the prophets"). Next, God's Spirit causes people to speak in tongues (2:4; 10:46; 19:6). The Spirit gives the apostles the power for many signs and wonders (2:43; 5:12; 6:8; 8:6,13b; 13:9-11; 14:3; 15:12; 16:18; 28:3-6), including healing people (3:7; 5:16b; 8:7; 9:40; 14:9-10; 19:11-12; 28:8-9). Also, the Holy Spirit gives boldness to preach the gospel (4:8,31,33; 6:10), speaks directly to people (8:29; 10:19-20; 11:12; 13:2; 15:28-29; 16:6-7; 20:23; 21:4), and prophesies (11:28; 21:9,11). There are a couple more things like comfort that are mentioned a few times. But for the most part, these are the works of the Holy Spirit in Acts. I'm not going to finish this with a disclaimer like: "but He doesn't work like this anymore" because it doesn't say anything like that. After I finish through the Bible I'm sure I will look at some commentaries on this, but until then, I am just relaying what is said. I know that I have never healed someone, spoken in tongues, cast out a demon, prophesied, or heard the Lord audibly talk to me, but I'm not going to say that it doesn't happen just because it hasn't happened to me. That would be ridiculous and nearsighted.
Now I want to talk about my favorite part in Acts, the way Paul preaches the gospel. Only a few times does it actually show him preaching. Much of the time, he is found reasoning with people and proving to them, through the Scriptures, that Jesus is the Christ (18:4,19; 19:8-9). I think that is so cool. He is sitting down there reasoning with these people and discussing things with them.
There are many other things in Acts concerning the church and what they looked like. Often the apostles are shown "preaching and teaching the word of God." Acts 20:17-38 is one of the most emotionally moving things that I have read in the Bible to this point. He is talking to the Ephesian elders and telling them that he probably won't ever see them again. He's giving them instruction about how to conduct themselves and the church. At the end of it they all knelt down to pray together and it says there was much weeping.
Anyway, there is a lot of good stuff in here but it is in little chunks since it is a narrative. I just wanted to give you the main point of what I thought Acts was giving and I believe that it is the work of the Spirit of the Lord. The rest of the things talked about in here (circumcision, salvation by grace alone, election, elders and deacons) will be discussed in the letters so I know I can discuss it in more detail with you there.
Soli Deo gloria