In the Book of Acts (17.23) Luke tells how Paul the apostle addresses the Athenians on the Areopagus and takes as his point of departure an inscription on an altar he saw in the city. This inscription, he says, ran as follows: “For an unknown god” (ἀγνώστῳ θεῷ).
All the other evidence for a cult of (an) unknown god(s) is later than Acts. In the 2nd cent. ce, Pausanias says that near the harbour of Phalerum (Athens) there were altars of gods named ‘unknown ones’ and of heroes (βωμοὶ δὲ θεῶν τε…