Plato’s concept of God seems to have been the central feature of his unwritten doctrine, based on Tubingen scholars arguing that it had to do with the One and Plato’s statement in the Timaeus, “Now to find the maker and father of the universe is hard enough, and even if I succeeded, to declare him to everyone is impossible” (28c). That you can only tell it to very few people lines up with what Plato said about his unwritten doctrine.
Plato seemed to have something monotheistic in opposition to the Greek pantheon since Socrates continually refers to “God” in a monotheistic way: one of the charges against Socrates was “not believing in the gods in whom the city believes, but in” new gods. (Apology 24-b-c).
With that in mind, here are a series of quotes that Andre Dacier thought were the most important for making the connection between Christianity and Platonism.
Dacier first cites a passage from the Epinomis, which, after describing heavenly bodies, says,
The most Divine WORD fram’d this Universe and render’d it visible. And that Man that is truly happy, first admires this WORD, and is afterward inflam’d with a desire of leaning all that can be known by a mortal Nature, being convinc’d that this is the only way, to lead a happy Life here below, and after Death to arrive at those places that are prepar’d for Vertue; where he shall be truly initiated and united with Wisdom; and always enjoy the most wonderful Discoveries (886c-d).
Next Dacier cites passages from two of Plato’s letters. The passage in letter six to a group of his followers says,
You must read my Letter all three together; and that you may profit by it, you ought to implore the Assistance of God the Soveraign Lord of all things that either are or shall be; and the Father of this Soveraign, who is the Cause of Beings. If we are truly Philosophers we shall know this God as clearly as Blessed Men are capable of knowing him (Letter 6, 323d).
And, finally, letter two says,
I must write in Enigmas, that is my Letter should be intercepted by Sea or Land, he that reads it may not be able to comprehend any thing. All things are round about their King; they exist by him, and he alone is the cause of good things: Second for second things, and third for third (Letter 2, 312e). [Yes, this passages sounds a lot like DC 88:41].
Dacier said that he and other Christian Platonists saw these passages not only as evidence that Plato believed in the Son of God or the Word, but also a trinity: “third for third.” While such a reading no doubt strains these passages, they are useful, I think, for shedding some light on Plato’s concept of deity and his unwritten doctrine.
First, the Epinomis (the first passage) is generally agreed not to have been written by Plato but by Philip of Opus, known as Plato’s scribe. It’s an addendum to Plato’s Laws, which is believed to be Plato’s last work. So Philip just tacked another dialogue on the end. What’s interesting about the Epinomis is that it begins with an attempt to define which God Plato meant, something Plato never did. “But I must say which god I mean,” the text says and then goes on to say he means Uranus or the universe (977a). Again, Plato never said any such thing, suggesting that this close follower wanted to pin down this central principal that his teacher had been so vague about.
Also interesting in the passage quoted about the Word or Logos. All other translations that I’ve read translate it as a principal (reason or law) and not a being, but the Logos as a being, though it developed among the Stoics, may have had its beginning in this passage.
Many scholars are also skeptical of the authenticity of letters 6 and 2, but perhaps like Epinomis they may have been written by close followers hinting at the unwritten doctrine. Or they may have been authentic because, one way or the other, they match closely what Plato said about his unwritten doctrine: what you write is only to remind you about the really important stuff. In letter two, Plato suggest his followers read the letter as some kind of religious rite, and both refer to the Father. Again, Plato said that the Father could only be made know to certain individuals, and both letters suggest that Plato was adhering to that principal.
So in sum, Socrates was accused of teaching new gods, Plato’s unwritten doctrine was mainly about God, and these hints at that doctrine suggest a Father, Son, and even a Logos. While claiming this to be Christianity is going too far, such statements seem to fit the basic Christian concept.
 Andre Dacier, The Works of Plato Abridg’d, 2 vols (London: A. Bell, 1701), 1:138-39.
 David Jones, “Pan’s Death and the Conspiracy of Logos: Plato’s Case against Myth,” in The Gift of Logos: Essays in Continental Philosophy, ed. David Jones (Newcastle upon Tyne: Cambridge Scholars), 32