I know my question seems weird, but in Plato's books Apology, Crito, and Phaedo (and probably in other books since I've read only these three and I am in the middle of the third one), when Socrates wants to swear in a god's name or talk about destiny or any similar thing, he says "God" and not "gods" (except maybe rarely).
Was he a monotheist? Or he was speaking in a "rhetorical" way? Or is it just the translation?
In several places, most notably the discussion of the "Allegory of the Cave" in The Republic, Plato's Socrates identifies the Ideal of the Good as the singular source of all good things in the universe. Plato's followers, the Neoplatonists, further identified this Idea of the Good with God, a perspective that was very influential on early Christian theology.
If the Neoplatonists were correctly interpreting Plato, and if Plato was accurately depicting Socrates, then we might reasonably conclude Socrates was a closet monotheist. He certainly speaks of the gods in a non-standard manner in Plato's dialogues --in fact, guiding youth to disbelieve in the Greek pantheon was one of the main charges against Socrates in the trial that led to his eventual execution.
The following is a set of excerpts from a lengthy section in Book II of The Republic where Socrates/Plato explicitly dismisses much of what is commonly taught about the gods as lies. The last statement in particular, although it references "every god", certainly appears to imply monotheism, if followed through to its unspoken conclusion (as there can only be one "fairest and best" thing conceivable).
Neither, if we mean our future guardians to regard the habit of quarrelling among themselves as of all things the basest, should any word be said to them of the wars in heaven, and of the plots and fightings of the gods against one another, for they are not true...
Then God, if he be good, is not the author of all things, as the many assert, but he is the cause of a few things only, and not of most things that occur to men. For few are the goods of human life, and many are the evils, and the good is to be attributed to God alone; of the evils the causes are to be sought elsewhere, and not in him...
Then we must not listen to Homer or to any other poet who is guilty of the folly of saying that two casks lie at the threshold of Zeus, full of lots, one of good, the other of evil lots...
Shall I ask you whether God is a magician, and of a nature to appear insidiously now in one shape, and now in another --sometimes himself changing and passing into many forms, sometimes deceiving us with the semblance of such transformations; or is he one and the same immutably fixed in his own proper image?...
Then it is impossible that God should ever be willing to change; being, as is supposed, the fairest and best that is conceivable, every god remains absolutely and for ever in his own form.
http://classics.mit.edu/Plato/republic.3.ii.html
Socrates generally swears by Dis, officially Zeus, but really 'the God', even though he uses conventional phrases like 'by the gods'. Plato opens texts with prayers to Pan, in the same way, and it clearly means 'All' and not the conventional god of that name. It is safe to hold that both were monotheists assigning their monotheistic god to a recognizable member of the pantheon for politeness sake.
This is backed up in a couple ways:
There is an argument in one of the dialogues (Euthyphro?) that the will of the gods should be singular, if it is to be a real guide to human action. It is left to be assumed that Socrates does not believe in multiple gods, who would therefore have multiple opinions of what piety was.
In the creation myths of the Gorgias and Timaeus, the Demiurge is singular, and this seems to be on purpose. It would be easier to include all of the things done (from brute work like making land to artistic flourishes like placing bones of given lengths in the fabric of the heavens to ensure proper harmonies resonate with human music) to a multitude of hands.
Any answer about Socrates faces the difficulty that we do not have any texts written by Socrates but only texts written about Socrates.
Texts about Socrates have been written by Plato, by Aristophanes and by Xenophon. The Socrates from Plato's late dialogues is certainly a fiction of Plato. While the Socrates of the early dialogues - Crito, Phaedo - and in particular the Socrates from Apology probably resembles the historical Socrates.
In the following I base my answer on the Socrates from Apology. Here Socrates tells us about his lifelong attempt to obey to the dictum of the God, i.e. to practice a philosophical life. He means the god Apoll from Delphi. Because the Pythia from the Delphic oracle said, speaking in the name of Apoll: Nobody is wiser than Socrates. And Socrates wants to find out the meaning of this dictum.
From this viewpoint Socrates serves one certain god, the god Apoll. On the other hand Socrates fights against the accusation not to believe in those gods which are venerated in Athens. And these are all of the Olympic gods, at least.
Hence the question whether the historical Socrates was a monotheist or a polytheist has to be left open.
The word "God " in Plato is usually a mistranslation of "ho theos", "the god". In at least one place this is demonstrably a reference to Apollo.
jobermark said, “There is an argument in one of the dialogues (Euthyphro?) that the will of the gods should be singular, if it is to be a real guide to human action. It is left to be assumed that Socrates does not believe in multiple gods, who would therefore have multiple opinions of what piety was.”
I suggest that Plato is not implying that there must be one god, but that the will of the gods is not a useful criterion for what to do. Rather, we must understand the Form of the Good.
jobermark said, “In the creation myths of the Gorgias and Timaeus, the Demiurge is singular, and this seems to be on purpose. It would be easier to include all of the things done (from brute work like making land to artistic flourishes like placing bones of given lengths in the fabric of the heavens to ensure proper harmonies resonate with human music) to a multitude of hands.”
This misrepresents the Timaeus. The gods participate in the shaping of the world. The Demiurge is merely the chief of them, to this extent like Zeus.
The short answer for this question is: Yes, Socrates was definitely a monotheist. He believed in one God.
To be more specific, Socrates claimed that he had contact with the Divine. In my opinion, there is a high probability that Socrates was even a prophet. You can recognize prophets based on several characteristics including:
Especially when reading The Apology, one can find a lot of these elements in Socrates' defence. In fact, as a muslim, I cannot find anything in Socrates' statements that contradicts my religion. Because Socrates' name is not mentioned explicitly in any Islamic source, I feel not comfortable to make such a claim. But I do think the chance that he is a monotheist and to go even further a Prophet, is very very likely.
