You said: "To be a true Christian one must be "born again" and that experience was not available or possible before Christ rose from the dead."
The New Birth was always made available to God's People - Deuteronomy 30:6
And the LORD your God will circumcise your heart and the heart of your offspring, so that you will love the LORD your God with all your heart and with all your soul, that you may live.
PAY ATTENTION to this dialogue: Gospel of John ch3
Now there was a man of the Pharisees named Nicodemus, a leader of the Jews.
2He came to Jesus at night and said, “Rabbi, we know that You are a teacher who has come from God. For no one could perform the signs You are doing if God were not with him.”
3Jesus replied, “Truly, truly, I tell you, no one can see the kingdom of God unless he is born again.
a”
4“How can a man be born when he is old?” Nicodemus asked. “Can he enter his mother’s womb a second time to be born?”
5Jesus answered, “Truly, truly, I tell you, no one can enter the kingdom of God unless he is born of water and the Spirit.
6Flesh is born of flesh, but spirit is born of the Spirit.
7Do not be amazed that I said, ‘You must be born again.’
8The wind blows where it wishes. You hear its sound, but you do not know where it comes from or where it is going. So it is with everyone born of the Spirit.”
9“How can this be?” Nicodemus asked.
10“Y
ou are Israel’s teacher,” said Jesus, “and you do not understand these things?