In addition to the twelve main gods and the innumerable lesser deities, ancient Greeks worshiped a deity they called Agnostos Theos, that is: the Unknown god. In Athens, there was a temple specifically dedicated to that god and very often Athenians would swear "in the name of the Unknown god" (Νή τόν Άγνωστον Ne ton Agnoston). Apollodorus, Philostratus and Pausanias wrote about the Unknown god as well. The Unknown god was not so much a specific deity, but a placeholder, for whatever god or gods actually existed but whose name and nature were not revealed to the Athenians or the Hellenized world at large.
Wikipedia | Unknown God