Our doctrines, then, appear to be greater than all human teaching; because Christ, who appeared for our sakes, became the whole rational being, both body, and reason, and soul. For whatever either lawgivers or philosophers uttered well, they elaborated by finding and contemplating some part of the Word. But since they did not know the whole of the Word, which is Christ, they often contradicted themselves. And those who by human birth were more ancient than Christ, when they attempted to consider and prove things by reason, were brought before the tribunals as impious persons and busybodies. And Socrates, who was more zealous in this direction than all of them, was accused of the very same crimes as ourselves. For they said that he was introducing new divinities, and did not consider those to be gods whom the state recognised. But he cast out from the state both Homer and the rest of the poets, and taught men to reject the wicked demons and those who did the things which the poets related; and he exhorted them to become acquainted with the God who was to them unknown, by means of the investigation of reason, saying, "That it is neither easy to find the Father and Maker of all, nor, having found Him, is it safe to declare Him to all." But these things our Christ did through His own power. For no one trusted in Socrates so as to die for this doctrine, but in Christ, who was partially known even by Socrates (for He was and is the Word who is in every man, and who foretold the things that were to come to pass both through the prophets and in His own person when He was made of like passions, and taught these things), not only philosophers and scholars believed, but also artisans and people entirely uneducated, despising both glory, and fear, and death; since He is a power of the ineffable Father, and not the mere instrument of human reason.
[Second Apology, Chapter X] 'Word' here is in the Greek Logos, which, of course, comes from the first chapter of John and also means 'Reason' -- hence St. Justin's argument that all the most noble pagan philosophers partly knew Christ. I've talked about St. Justin's philosophical background previously.
ADDED LATER: It is perhaps worth pointing out that the quotation that St. Justin gives is from Plato's Timaeus, and probably done from memory; it is actually Timaeus, not Socrates, speaking (although Socrates seems to agree with it).