Why is Plato concerned about the difference between mere belief and true knowledge, particularly concerning the issues of justice? How does this display his view of rhetoric?

Plato questioned if there could actually be a true art of rhetoric, one founded on a love of wisdom and a knowledge of justice. As he listed and suggested there can be such an art. As it would consist of a thorough knowledge of the different types of human souls, as well as through the knowledge to make arguments that would appeal to each type of soul. Plato presents us with two types of rhetoric as one being evil and one being virtuous. He recognized the mere power of persuasive language and how the sophists had mastered it.
He sees the sophists as how they claim to teach about justice while having no real knowledge of justice itself. Justice, for Plato, is represented as knowledge that requires deep study and dedication. It is to be discovered through deep thought, study, and religious virtue. This displays his views of rhetoric through kairos, and the power of language.