Nicodemus obviously couldn't understand what Jesus was saying,

        Jesus was talking about a spiritual rebirth,
                  but Nicodemus was thinking of a physical rebirth.
              This was obvious by Nicodemus' reply as to how can a man enter a second time into his mother's womb.

In John, chapter three, Nicodemus, a ruler of the Jews, came to    Jesus by night to ask about the miracles, "For no man can do these miracles that thou doest, except God be with Him."

          Jesus' reply seemed out of context with his question.
     Jesus said, "Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God."

Man had proven that he, himself couldn't obey the law,
        for the letter kills but Spirit gives life.

        The law was ministering death to man,
                but God would minister life through the living Word.