What was Nicodemus supposed to know from the Old Testament about being born again?
“Nicodemus said to Him, "How can these things be?" Jesus answered and said to him, "Are you the teacher of Israel and do not understand these things?” John 3:9--10 NASB
So what OT verses was Jesus referring to in anticipation of the born again experience to come?
Jesus was not referring to any OT verses in anticipation of the born again experience to come, because He said to Nicodemus:
" No one has gone up into heaven, but there is One who came down from
heaven, the Son of Man [Himself—whose home is in heaven]."( John 3:13
To inform you of such heavenly things.
"Are you the teacher of Israel and do not understand these things?” Nicodemus was a scholar of religious law and a member of the Sanhedrin ,the High Court, and as a teacher he new many prophecies regarding the Messiah, such as:
1/Born of a virgin: Isaiah 7:14 (Matthew 1:18-25)
2/ Not believed in: Isaiah 53:1 (John 12:37-38)
3/ Heir to throne of David and his kingdom: Isaiah 9:7 (Matthew 1: 6-17)
4/ Jesus was born in Bethlehem: Micah 5:2 (Luke 2:1-7)
So What exactly, Nicodemus did not understand?
Read John 3:1-8 NASB
1 "Now there was a man of the Pharisees, named Nicodemus, a ruler of
the Jews; 2 this man came to Jesus by night and said to Him, “Rabbi,
we know that You have come from God as a teacher; for no one can do
these signs that You do unless God is with him.” 3 Jesus answered and
said to him, “Truly, truly, I say to you, unless one is born again he
cannot see the kingdom of God.”
Nicodemus *said to Him, “How can a man be born when he is old? He
cannot enter a second time into his mother’s womb and be born, can
he?” 5 Jesus answered, “Truly, truly, I say to you, unless one is born
of water and the Spirit he cannot enter into the kingdom of God. 6
That which is born of the flesh is flesh, and that which is born of
the Spirit is spirit. 7 Do not be amazed that I said to you, ‘You must
be born again.’ 8 The wind blows where it wishes and you hear the
sound of it, but do not know where it comes from and where it is
going; so is everyone who is born of the Spirit.”(John 3:1-8 NASB)
Nicodemus said to Him, “How can these things be?(John 3:9)
Jesus replied to him:
John 3:10-13 NASB
“Are you the teacher of Israel and do not understand these things? 11
Truly, truly, I say to you, we speak of what we know and testify of
what we have seen, and you do not accept our testimony. 12 If I told
you earthly things and you do not believe, how will you believe if I
tell you heavenly things? 13 No one has ascended into heaven, but He
who descended from heaven: the Son of Man. "
Jesus , basically was telling Nicodemus that He who was living with his heavenly father and descended from heaven (Verse 13) was in a position to instruct him, how to enter the Kingdom of God ,but since you (Nicodemus) do not accept our (Jesus) testimony about heavenly things ,(Verse 11-12) then here was no other way to obtain the knowledge he wanted, because no man has accented into heaven to obtain such heavenly information, except the "Son of Man"who descended from heaven. (verse 13)
Despite his knowledge and the fact that he had seen Jesus perform many miracles, yet this did not convince him that Jesus was the Messiah , accept his teachings, repent , get baptized and exercise faith in Jesus so that he could be reborn again . The 120 disciples of Jesus at Pentecost 33 C.E. were the first persons to experience this new birth.
The scriptures do not say if Nicodemus later became a follower of Christ.
However it is possible that Nicodemus later did become a follower of Christ, for he humbly acknowledged : “Rabbi, we know that You have come from God as a teacher." And the fact that later Nicodemus with Joseph of Arimathea, showed up and took the body of Christ and wrapped it with the spices in linen cloths, according to the burial custom of the Jews." (Verse John 19:40 NRSV)