In addition to the twelve main Gods and the innumerable lesser deities, ancient Greeks used to worship an Unknown God. In Athens, there was a temple specifically dedicated to that God and very often Athenians used to swear "in the name of the Unknown God" (Νή τόν Άγνωστον). Apollodorus, Philostratus and Pausanias wrote about that God as well.
When the Apostle Paul visited Athens he saw an altar with an inscription dedicated to that God so when he gave his speech on the Areopagos he told the crowd that he was there to talk about that same Unknown God.
