We know that Nicodemus was a believer so when Jesus said to Nicodemus "You must be born again" had he at that time already been born from above"? Remember, Nicodemus asks "How can these things be?" so he did not understand and Jesus was not telling him that the new birth had already taken place in his life; just that he must be born again.
It appears that Jesus was speaking of a discrete event, not a process.
It's semantics, isn't it? His name had been written in the Book of Life by God from before the foundation of the world and no man could erase it. Yet Nicodemus didn't know that until God chose a time to reveal it to him.
God gives understanding, and with understanding comes faith and repentance.
"And when the Gentiles heard this, they were glad, and glorified the word of the Lord: and as many as were ordained to eternal life believed." -- Acts 13:48
That's why faith doesn't save; grace saves. Our faith is the means which God employs to make us understand that Christ took the punishment rightly due us.
Once I understood that, my faith was a lot stronger.