Nicodemus recognized Jesus' authority without understanding the concept of being born again. As "a member of the Jewish ruling council", he had great Biblical knowledge, but as we can see right here, knowledge alone cannot save. I believe that it was by faith that Nicodemus recognized Jesus' authority. As a top Biblical leader when this information was very newly revealed, he likely thought that he needed the knowledge as well. How else could he even be allowed to teach it to others? It's good to ask questions. While looking up some information about the differences between faith and knowledge, I found this and it's excellent:
Tim Stratton says, "In conclusion, I agree... that Christians aren’t only people of faith, but people of a faith that is justified by logic, data, and evidence, rather than by presuppositions and baseless personal opinions (which seems to be the case with many atheists).[4] When Christians attain this knowledge of God and Scripture, their faith will grow extremely strong which will be evident in their prayer lives, their Christian walk, and their commitment to fulfilling the Great Commission. Evangelism will quickly transform from something that Christians are scared of into something they eagerly anticipate. When one possesses knowledge, he has things he knows to share with others. When Christians share their faith, rooted in knowledge of reality, these evangelical encounters will quickly multiply with awesome results!
On the side of Truth (John 18:37)"