H.V. Morton remarks in his book In The Steps Of Saint Paul that in appealing to their altars of the "Unknown God", St. Paul's teaching, as always, was tailored to his audience. He didn't quote the Hebrew Scriptures, which would have meant little to his listeners, but instead gave them something of their own: "It was an excellent beginning. It had the local touch, the right note of something surprising to follow. To everyone who listened to St. Paul, the altars inscribed TO AN UNKNOWN GOD were, of course, a commonplace. Everyone knew the story of the plague that visited Athens in the sixth century before Christ, and how, after sacrifices had been made to every known god and the plague continued, the services of the Cretan prophet, Epimenides, were requested. He drove a flock of black and white sheep to the Areopagus and allowed them to stray from there as they liked, waiting until they rested of their own free will: and on those spots were the sheep sacrificed 'to the fitting god'. The plague ceased, and it became the custom, not in Athens alone, to erect altars to unknown deities."
Below is footage from the June 29, 2007 Great Vespers Service for the Feast of the Holy Twelve Apostles at Mars Hill (Areopagus) in Athens, where the Apostle Paul preached to the Athenians. For photos from the ceremony in 2010, see here.
A recreation of the speech of the Apostle Paul can be seen here.