What does John 2:25 mean?

Jesus didn't need anyone to educate Him about human nature, or other people's thoughts. He knew exactly what each person needed to know, and what they needed to hear. This is why verse 24 says He didn't trust all of the people claiming to follow Him in Jerusalem. He knew many of them were just following to see more miracles. Jesus will prove His ability to reach people right where they need it over and over.

With the woman at the well, an outcast among outcasts, He uses acceptance (John 4:23–24). With someone like Nathanael, He uses dry humor (John 1:47). With Nicodemus, a legalistic and educated man, He uses deep thinking (John 3:10). The comment that Jesus knew "what was in man" is the bridge connecting the cleansing of the temple to the conversation with Nicodemus in chapter 3. Immediately after saying that Jesus knows all about man, and what was in man, John 3:1 says that "a man of the Pharisees" came to see Jesus. Part of that conversation will be Jesus proving that Nicodemus isn't as spiritually wise as he thinks he is.