John 3:6

Here, not in John 3:5, Jesus clarified that there are two types of birth, one physical and one spiritual. "Flesh" again refers to human nature (cf. John 1:14). The Holy Spirit gives people spiritual life. We are spiritually dead in sin until the Spirit gives us spiritual life. Jesus had been speaking of a spiritual birth, not a physical one. Nicodemus should not have marveled at the idea that there is a spiritual birth as well as a physical birth since the Old Testament spoke of it (cf. Psalms 87:5-6; Ezekiel 36:25-28). It revealed that entrance into the kingdom is a spiritual matter, not a matter of physical descent or merit. This was a revelation that most of the Jews in Jesus’ day, including Nicodemus, missed.