On Pentecost, which, oddly, is the day I'm writing this, the disciples are all sitting around harmoniously (which is code for arguing vociferously) when the room fills up with a sound from heaven as of a rushing mighty wind (v. 2). Cloven tongues like as of fire (v. 3) appear and perch on their heads. This causes them to start speaking in other languages. Cooler than the Universal Translators on Star Trek, not as good as a babelfish.
Now, what you need to know about Jerusalem was that, much like today, it was a cosmopolitan, polyglot city, filled with Jews from all over the Mediterranean. They hear about this new phenomenon, and gather curiously. They're all amazed to hear their own languages coming out of the guys' mouths. They just can't figure it out, until someone scoffs that (v. 13) which makes total sense to me, because I used to live in Japan, where everybody speaks English, but only after 3 beers. They even have a word for it: nomunication.
Peter, who has clearly never seen Mad Men, insists they aren't drunk because it's only 3 o'clock. No, rather, this is fulfilling a prophesy from the book of Joel, about how at the end of the world, everyone will be a prophet. There will be some signs, like lunar eclipses and volcanoes, and anyone who repents will be saved. He chastises them for killing Jesus and references how he's David's great-something grandchild. It convinces the Jews, who ask what they can do now. It's pretty simple really: Repent, and be baptized every one of you in the name of
Jesus Christ for the remission of sins, and ye shall receive the
gift of the Holy Ghost (v. 38) though hopefully the gift will not be in the form of a tongue on your forehead. Three thousand people take him up on the offer.
The disciples do some more magic tricks that convince their new converts they've made the right choice, so they sell all their possessions and share all that remains and more and more people are convinced by them and join up.
Well, that was a chapter that started off awesomely and finished disappointingly.