I've really been hung up lately on Paul's preaching in Athens. I'm not sure why, but I've really dug in here for a while, and I keep finding new things.
On the whole, the passage teaches us a very valuable insight: whether or not we know it in so many words, we all worship something. Each of us chases a god or God (even the ones who say they don't worship anything at all). Some, if not most, don't know this particular god by name, and so their personal shrine may read "TO AN UNKNOWN GOD." Anything can fit on that pedestal and claim the inscription: wealth, fame, power, knowledge, self, family, church, job, team, gadget, political party, you name it. And every day it gets worshiped at its altar.
The Athenians of Acts 17:16-34 were probably just trying to hedge their bets. Most likely Paul found himself surrounded by altars to every deity in the Greco-Roman pantheon, from Zeus down to Hecate. But a good polytheist can't afford to anger any of the gods and goddesses, so they also had an altar to the unknown god, essentially saying, "In case we missed you, whoever you are, this one's for you. Please don't smite me." Paul explained to them he knew that God -- and He alone could bring salvation for the soul.
Which god/God do you worship? Do you realize what you're doing, or are you, as Paul says, "ignorant of the very thing you worship" (v. 23)? Only one God is truly knowable: the God who has acted in history to bring salvation through a personal relationship with human beings. Now that's a God worth knowing!