What was the Unknown God did Greeks had and if it is unknown God, why it is written in Capitel Letters in the Acts?(Acts. 17.23) "I even found an altar with this inscription: to an unknown god. So you are ignorant of the very thing you worship"
As you may know, the original manuscripts would have been written in uncials, i.e., all capital letters. Hence, the all caps “TO THE UNKNOWN GOD” in any English translation is the translator’s personal decision.
According to Pausanias, “a Greek traveler and geographer of the second century AD,”1 who wrote a work entitled Ἑλλάδος Περιήγησις (“Description of Greece”),2
The Athenians have also another harbour, at Munychia, with a temple of Artemis of Munychia, and yet another at Phalerum, as I have already stated, and near it is a sanctuary of Demeter. Here there is also a temple of Athena Sciras, and one of Zeus some distance away, and altars of the gods named Unknown, and of heroes, and of the children of Theseus and Phalerus...
Ἔστι δὲ καὶ ἄλλος Ἀθηναίοις ὁ μὲν ἐπὶ Μουνυχία λιμὴν καὶ Μουνυχίας ναὸς Ἀρτέμιδος, ὁ δὲ ἐπὶ Φαληρῷ, καθὰ καὶ πρότερον εἴρηταί μοι, καὶ πρὸς αὐτῷ Δήμετρος ἱερόν. ἐνταῦθα καὶ Σκιράδος Ἀθηνᾶς ναός ἐστι καὶ Διὸς ἀπωτέρω, βωμοὶ δὲ θεῶν τε ὀνομαζομένων Ἀγνώστων καὶ ἡρώων καὶ παίδων τῶν Θησέως καὶ Φαληεροῦ·
The origin of or reason for such altars is not known with absolute certainty, but Meyer3 cites an account by Diogenes Laertius that may provide a clue,4
Hence, when the Athenians were attacked by pestilence, and the Pythian priestess bade them purify the city, they sent a ship commanded by Nicias, son of Niceratus, to Crete to ask the help of Epimenides. And he came in the 46th Olympiad, purified their city, and stopped the pestilence in the following way. He took sheep, some black and others white, and brought them to the Areopagus; and there he let them go whither they pleased, instructing those who followed them to mark the spot where each sheep lay down and offer a sacrifice to the local divinity. And thus, it is said, the plague was stayed. Hence even to this day altars may be found in different parts of Attica with no name inscribed upon them, which are memorials of this atonement.
Τότε καὶ Ἀθηναίοις [τότε] λοιμῷ κατεχομένοις ἔχρησεν ἡ Πυθία καθῆραι τὴν πόλιν: οἱ δὲ πέμπουσι ναῦν τε καὶ Νικίαν τὸν Νικηράτου εἰς Κρήτην, καλοῦντες τὸν Ἐπιμενίδην. καὶ ὃς ἐλθὼν Ὀλυμπιάδι τεσσαρακοστῇ ἕκτῇ ἐκάθηρεν αὐτῶν τὴν πόλιν καὶ ἔπαυσε τὸν λοιμὸν τοῦτον τὸν τρόπον. λαβὼν πρόβατα μέλανά τε καὶ λευκὰ ἤγαγε πρὸς τὸν Ἄρειον πάγον: κἀκεῖθεν εἴασεν ἰέναι οἷ βούλοιντο, προστάξας τοῖς ἀκολούθοις ἔνθα ἂν κατακλίνοι αὐτῶν ἕκαστον, θύειν τῷ προσήκοντι θεῷ: καὶ οὕτω λῆξαι τὸ κακόν. ὅθεν ἔτι καὶ νῦν ἔστιν εὑρεῖν κατὰ τοὺς δήμους τῶν Ἀθηναίωι βωμοὺς ἀνωνύμους, ὑπόμνημα τῆς τότε γενομένης ἐξιλάσεως.
The sacrifices were made «τῷ προσήκοντι θεῷ»—“to the concerned god.” It seems, then, that the Greeks could not explain the cause of the pestilence and therefore did not attribute it to a particular god, e.g., Apollo, Zeus, etc. In order to stay the pestilence by appeasing the god by sacrificing to it, they simply made altars inscribed to “the unknown god,” leaving them anonymous (viz., βωμοὺς ἀνωνύμους).
Diogenes Laertius. Lives of Eminent Philosophers. Trans. Hicks, R. D. Ed. Hicks, R. D. Cambridge: Harvard UP, 1925.
Meyer, Heinrich August Wilhelm. Critical and Exegetical Handbook to the Acts of the Apostles. Trans. Gloag, Paton James. Ed. Dickson, William P. 2nd ed. New York: Funk, 1889.
Pausanias. Description of Greece. Trans. Jones, W. H. S. Vol. 1. London: Heinemann, 1918.