How we cite our quotes:
Quote #1
The just man then, as it seems, has come to light as a kind of robber, and I'm afraid you learned this from Homer. (334a)
Socrates is taking Homer to task for portraying the just man in a negative light. This is pretty early on in the dialogue, which just goes to show that Socrates is up in arms about poetry and its portrayal of justice from the get-go.
Quote #2
Then for many, Polemarchus—all human beings who make mistakes—it will turn out that to be just to harm friends, for their friends are bad and just to help enemies, for they are good. So we shall say the very opposite of what we asserted Simonides means. (334d-e)
As Socrates and his pals try to make sense of this slippery term "just," Socrates, in his typical way, shows how nothing is ever simple: since people constantly make mistakes, Socrates points out that we can't rely on most people's judgment to make sense of concepts like "good," "bad," and "just."
Quote #3
"But as to what each [justice and injustice] does with its own power when it is in the soul of a man who possesses it and is not noticed by gods and men, no one has ever, in poetry or prose, adequately developed the argument that the one is the greatest of evils a soul can have in it, and justice the greatest good." (366e)
Socrates is laying out his agenda here. He wants to show how beneficial and great justice can be for an individual person, regardless of whether anyone, human or divine, notices that this individual is just. Philosophy to the rescue.