Nicodemus said to Yeshua (Jesus): "Rabbi, we know that thou art a teacher come from God: for no man can do these miracles that thou doest, except God be with him." Yeshua answered him: "Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God". Nicodemus didn't understand His words and asked Yeshua: "How can a man be born when he is old? can he enter the second time into his mother's womb, and be born?" Yeshua answered him: "Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God. That which is born of the flesh is flesh; and that which is born of the Spirit is spirit. Marvel not that I said unto thee, Ye must be born again. The wind bloweth where it listeth, and thou hearest the sound thereof, but canst not tell whence it cometh, and whither it goeth: so is every one that is born of the Spirit." Nicodemus asked Him again: "How can these things be?" Yeshua aswered him: "Art thou a master of Israel, and knowest not these things? Verily, verily, I say unto thee, We speak that we do know, and testify that we have seen; and ye receive not our witness. If I have told you earthly things, and ye believe not, how shall ye believe, if I tell you of heavenly things?" We can read about this in Besora Tova of John, chapter 3.

Why does Yeshua say so: "Art thou a master of Israel, and knowest not these things?" What could Nicodemus know from Tanakh?

Isaiah 44:3 For I will pour water upon him that is thirsty, and floods upon the dry ground: I will pour my spirit upon thy seed, and my blessing upon thine offspring:

"As I will pour water on the thirsty: Just as I pour water on the thirsty, so will I pour My spirit on your seed." (Commentary of Rashi http://www.chabad.org/library/bible_cdo/aid/15975/showrashi/true)

"Then will I sprinkle clean water upon you, and ye shall be clean: from all your filthiness, and from all your idols, will I cleanse you. A new heart also will I give you, and a new spirit will I put within you: and I will take away the stony heart out of your flesh, and I will give you an heart of flesh. And I will put my spirit within you, and cause you to walk in my statutes, and ye shall keep my judgments, and do [them]." (Ezek 36:25-27)

"clean water: I will grant you atonement and remove your uncleanliness by sprinkling purification water, which removes [even the highest degree of defilement,] the defilement caused by a corpse." (Rashi http://www.chabad.org/library/bible_cdo/aid/16134/showrashi/true)

To be born again is:

1) Matt 3:8 Bring forth therefore fruits worthy for repentance:

2) To know Bible;

3) To believe in Messiah Yeshua