When Jesus Christ challenges Nicodemus, a Jewish Pharisee, in relating with God, He says, “Ye must be born again.” (Jn 3:7)
Since this is in the New Testament, and we never hear it taught from the Old, it’s easy to think that being born again is relatively novel, something Moses, David and Abraham knew nothing about.
But Christ is speaking before the Cross, before He dies and rises again, so nothing has actually changed since Mount Sinai, when God revealed His Law, or really even since Adam. There’s no New Testament scripture at this point in time, yet Christ acts as if Nicodemus should already know about being born again, as if it’s obvious from the Old Testament. (Jn 3:10) How significant! If we don’t see being born again in the Old Testament like Jesus expects, what makes us think we understand it?
In a similar encounter, Christ challenges a woman and says something just as striking. “Ye worship ye know not what: we know what we worship: for salvation is of the Jews.” (Jn 4:22) He’s saying that if we don’t understand the salvation presented in the Old Testament, the oracles of God committed to the Jews (Ro 3:1-2), then we don’t understand salvation at all; we’re worshiping in ignorance. Not a good place to be.
In a third encounter, Christ tells an equivalently insightful story of a rich man suffering in Hell, concerned that his family will follow after him into its flames. He asks Abraham to send an acquaintance back from the dead to warn them. Abraham says, “They have Moses and the prophets; let them hear them.” (Lk 16:29) The claim is that Old Testament scriptures are a sufficient witness of the gospel. But the rich man pleads, convinced that the Old Testament is insufficient; if someone they knew rose from the dead to warn them, then they would repent and be saved. But Abraham is firm: “If they hear not Moses and the prophets, neither will they be persuaded, though one rose from the dead.”
Not only is the Old Testament a sufficient witness of the gospel, it is so overwhelmingly sufficient that if one isn’t convinced through it, then nothing will convince them.
Salvation is of the Jews: accomplished by Christ, a Jew, and revealed by and through Jews, God’s chosen people, in the scriptures God has transmitted to us all through them. This doesn’t mean we have to become Jewish in order to be right with God (1Co 7:18-20), but it does mean that the gospel of the New Testament is exactly the same as the gospel of the Old Testament. If the gospel we believe in isn’t an Old Testament gospel, then it’s a false one. (Ga 1:8)