John 2:23 Now when he was in Jerusalem at the passover, in the feast day, many believed in his name, when they saw the miracles which he did.
John 2:24 But Jesus did not commit himself unto them, because he knew all men,
John 2:25 And needed not that any should testify of man: for he knew what was in man.
This account occurs just after Jesus cleared the temple of the moneychangers and those selling sacrificial animals. He had also already turned water into wine at a wedding in Cana. Yet already there are those that are believing on him because of his boldness and the miracles they witnessed Him preform.
Is this true belief? No! Jesus knew it wasn’t true belief, because He knew the hearts of man. He created man so he knows how we think. True belief occurs when you get past what you can see and think with your head, but when your heart fully embraces as true.