The Unknown God or Agnostos Theos (Ancient Greek: Ἄγνωστος Θεός) 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" (Νὴ τὸν Ἄγνωστον Ne ton Agnoston). Apollodorus, Philostratus and Pausanias wrote about the Unknown God as well.
According to the book of Acts, contained in the Christian New Testament, when the Apostle Paul visited Athens, he saw an altar with an inscription dedicated to that god (possibly connected to the Cylonian affair), and, when invited to speak to the Athenian elite at the Areopagus, gave the following speech:
Acts 17:22-31 (WEB): 22 Paul stood in the middle of the Areopagus, and said, “You men of Athens, I perceive that you are very religious in all things. 23 For as I passed along, and observed the objects of your worship, I found also an altar with this inscription: ‘TO AN UNKNOWN GOD.’ What therefore you worship in ignorance, this I announce to you. 24 The God who made the world and all things in it, he, being Lord of heaven and earth, doesn’t dwell in temples made with hands, 25 neither is he served by men’s hands, as though he needed anything, seeing he himself gives to all life and breath, and all things. 26 He made from one blood every nation of men to dwell on all the surface of the earth, having determined appointed seasons, and the boundaries of their dwellings, 27 that they should seek the Lord, if perhaps they might reach out for him and find him, though he is not far from each one of us. 28 ‘For in him we live, and move, and have our being.’ As some of your own poets have said, ‘For we are also his offspring.’ 29 Being then the offspring of God, we ought not to think that the Divine Nature is like gold, or silver, or stone, engraved by art and design of man. 30 The times of ignorance therefore God overlooked. But now he commands that all people everywhere should repent, 31 because he has appointed a day in which he will judge the world in righteousness by the man whom he has ordained; of which he has given assurance to all men, in that he has raised him from the dead.”
Because the Jewish God could not be named, it is possible that Paul's Athenian listeners would have considered his God to be "the unknown god par excellence". His listeners may also have understood the introduction of a new god by allusions to Aeschylus' The Eumenides; the irony would have been that just as the Eumenides were not new gods at all but the Furies in a new form, so was the Christian God not a new god but rather the god the Greeks already worshipped as the Unknown God. His audience would also have recognized the quotes in verse 28 as coming from Epimenides and Aratus, respectively.
There is an altar dedicated to the unknown God found in 1820 on the Palatine Hill of Rome. It contains an inscription in Latin that says:
SEI·DEO·SEI·DEIVAE·SAC G·SEXTIVS·C·F·CALVINVSPR DE·SENATI·SENTENTIA RESTITVIT
Which could be translated into English as: "Either for a god or a sacred goddess, Caius Sextius Calvinus, son of Gaius, praetor by order of the Senate restored this."
The altar is currently exhibited in the Palatine Museum.