IF THEY BELIEVE, ARE THEY SAVED?

I was reading this morning in John 2, at the end of the chapter.  It says, beginning in verse 23,

"Now when He was in Jerusalem at the Passover, during the feast, many believed in His name, observing His signs which He was doing.  But Jesus, on His part, was not entrusting Himself to them, for He knew all men, and because He did not need anyone to testify concerning man, for He Himself knew what was in man."

 Here are a few things I have observed:

1) People believed in His name as they observed His 'signs'.
2) Jesus did not 'entrust Himself to them'.
3) He did not do so because of what he knew about man.

A few questions arise immediately after making those three observations.

1) What does Jesus know about man that caused Him not to entrust Himself to them?
2) What does it mean to have Jesus entrust Himself to someone?
3) If they believed, why does the passage carry a negative feeling with it that sort of implies that those who believed were not saved?

Just some thoughts for you to consider...