Known as the introduction or prologue, Luke sets the stage for the rest of the book. Recall that in his intro to his Gospel account, Luke also addresses Theophilus, and states that he’s undertaken careful research and compiling of the testimony so he could present it in an orderly way. He continues that approach here.
And, we hear Luke referring to “the first book” (literally, “the first word/account”, in Greek ton prōton logon, a form of “logos”), which of course is the Gospel account. So this is now “book two” of the same story.
1:1 Notice the language of “began to do and teach”. In the Greek language, word order is used to emphasize things, and here there is a bit of an emphasis on “began”. The implication is that Jesus is not in fact absent, even though Acts begins with the Ascension. He is still doing and teaching, the Holy Spirit continuing to be the active agent of Jesus’ work.
Even now, the disciples seem to be focused on the earthly kingdom they were expecting. They still don’t quite “get it”. Yet Jesus continues to patiently teach, then do!
1:8 It’s an important note that this is eyewitness testimony; the Apostles are first-hand witnesses to the things that we hear through Luke, who himself was a first-hand witness to some things (e.g. Acts 16:11-17, 2 Timothy 4:11) and had first-hand sources for other things as he compiled his accounts.
Also notice the “fanning out” of the action again. Remember the “hourglass” shape of Luke-Acts? It starts “wide” then narrows into Jerusalem, and then goes out again from Jerusalem. What Jesus promises here is fulfilled as we travel through Acts and beyond, even into the Church today.
1:11 Jesus will not return in the “hiddenness” of the Incarnation. When He returns again, everyone will know it.
The Apostles and the other disciples (now about 120 in total - vv. 14-15) are active in their waiting for the promise of Jesus to be fulfilled. They don’t simply sit around doing nothing, but are devoting themselves to prayer (v. 14) and meeting together, as well as doing the work of replacing Judas in the Twelve, so that the number would continue to be fulfilled properly.
1:13 The upper room in Jerusalem could very well be the same one they observed the Last Supper in. It becomes, as it were, their base of operations, their “church building” until the day of Pentecost (and possibly beyond that).
1:21 Notice the qualifications: a man who has been with them the whole time of Jesus’ ministry, who can be a witness together with them.
1:26 Though votes would become a part of selecting elders/pastors/bishops later on, here the decision is made by casting lots. Most people would call this “chance”, since it’s akin to a rolling of the dice or flipping a coin. But the Apostles entrusted this action to God in faith. This has become a bit of a model for choosing a pastor in a congregation; it’s not wrong to vote, but it also could be just as valid to have 2 or 3 candidates who would also meet the qualifications, and then pray and cast some kind of lot for it.
Through this selection, Matthias becomes a “capital-A” Apostle with the other 11.