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. That which is born of the flesh is flesh, and that which is born of the Spirit is spirit." (John 3:5-6)
Religious people like Nicodemus, who was a Pharisee and a a ruler of the Jews, are especially offended by the gospel of Jesus Christ. Why? Because they have a high regard for themselves and diminish the holiness of God. They understand that they are sinners, but believe that by their religious education, offerings and sacrifices, they have been reformed and are acceptable to God.
I grew-up in a religious family that regularly attended a main-line denominational church. I was baptized, confirmed and an active member of the church. I read the Bible (occasionally) and believed in the triune God. Yes, of course, I was a sinner because nobody is perfect. But I was more religious than most of my friends and assumed that, if there really was a literal heaven and hell, I would be judged on a sliding scale to be good enough to go to heaven. I was religiously reformed.
Then one day I heard a preacher say that God is holy, holy, holy and that His acceptable requirement for me is perfection. Not religious reformation, but transformation... You must be spiritually "born again"! Those who have come to truly believe the gospel of Jesus Christ are not just reformed, religious sinners, they are transformed; Therefore if anyone is in Christ, he is a new creature; the old things passed away; behold new things have come.
(2 Corinthians 5:17)
Our religious beliefs and conduct, our baptism, church attendance, Bible reading and all other good works are pleasing to God only after you have first been transformed by faith alone in the gospel of Jesus Christ. Transformation via our justification is needed before reformation via our sanctification. You must be born again! Maranatha!