An altar to an unknown god. How strange! The Greeks worshiped many, many gods, so that in Athens it is said that there were more gods than people. But what if they had forgotten a god here or there? What if one had been lost from their records? Surely they did not want to offend some deity or incur his wrath upon themselves by neglecting to worship him. In order to cover all the bases and make sure no god was being left out, they included also this altar to an unknown god.
The apostle Paul used this altar to an unknown god to explain to the people that in the midst of their many idols, the most unknown god in Athens was the one true God.