‘Now when he was in Jerusalem at the Passover, during the feast many believed on his name when they saw the signs which he did, but Jesus did not trust himself to them because he knew all men, and because he did not need that anyone should testify to him concerning man, for he himself knew what was in man.'
Two interpretations are possible for these verses.
The first sees this as the completest rebuttal of those who think people will believe because of miracles that we could have. That is what these people did, runs this view, they believed in Him because they saw miracles. But Jesus knew how unreliable such faith was, and that the only faith worth having is that which is based on an inner certainty of Who Jesus is and a full response to Him based on that certainty. That is what the ‘signs' mentioned by John are meant to accomplish, the giving of understanding (John 20:31). These people did not understand.
‘Jesus did not trust himself to them.' This would then mean that He would not encourage them to become disciples until He had more evidence of their genuineness. He was never concerned about numbers and popularity, and was quite happy to limit their number (compare John 6:66).
The second possibility is that their faith was genuine, but that there was a danger of them seeking, in their enthusiasm, to press Him into Messianic activity outside His purposes. Compare how later He withdraws from the crowd who would make Him a king (John 6:15). Thus He does not take them under His wing, and does not wish to be too closely involved with them.
But the fact that this comes before the incident of Nicodemus whose faith also was lacking must be seen as supporting the first suggestion, for Nicodemus at this stage illustrates one whose understanding is lacking. He too came because he had seen signs (John 3:2), but did not understand their significance.
It is interesting that John does not mention any specific miracles here. He just assumes them. They were an important evidence of Jesus' compassion, and of His status, but they were not seen by John as relevant to his purpose. He is not citing them as ‘evidence'. He is making clear that Jesus knew men and women through and through. ‘He knew what was in man'. For Jesus does not want those who merely respond to miracles. He wants only those who are genuine in seeking Him with all their hearts.