The Unknown God or Agnostos Theos is a theory by Eduard Norden first published in 1913 that proposes, based on the Christian Apostle Paul's Areopagus speech in Acts 17:23, that in addition to the twelve main gods and the innumerable lesser deities, ancient Greeks worshipped a deity they called "Agnostos Theos"; that is: "Unknown God", which Norden called "Un-Greek". In Athens, there was a temple specifically dedicated to that god and very often Athenians would swear "in the name of the Unknown God". Apollodorus, Philostratus and Pausanias wrote about the Unknown God as well.

The Areopagus sermon refers to a sermon delivered by Apostle Paul in Athens, at the Areopagus, and recounted in Acts 17:16–34. The Areopagus sermon is the most dramatic and most fully-reported speech of the missionary career of Saint Paul and followed a shorter address in Lystra recorded in Acts 14:15–17.