Why did the Athenians have an altar to an unknown God if they already had many gods?
The Athenians were polytheistic and sought to cover every possibility in worship. Having many gods and goddesses, they added an altar to an unknown God just in case they had omitted one, showing they did not know the true God of creation.
Les explains that Paul, as he passed by and beheld their devotions—their idolatrous worship—found an altar with the inscription “to the unknown God.” The Athenians already had many gods and goddesses and all their worship, yet just in case they were missing one, they set up a special altar dedicated to the unknown God. Les compares this to a man who once asked him, during financial hardship, whether he had thought about praying to St. Jude; the man said he was a firm believer you “plug every hole.” That, Les says, is the way these Athenians were. Despite all their idols, they knew nothing of the true God or the God of creation.
Which is his body, the fulness of him that filleth all in all.