John 3:3
Jesus answered and said unto him, Verily, verily, I say unto thee,
Except a man be born again, he cannot see the kingdom of God.
In The Gospel of John chapter 3, we have the record of Nicodemus coming to the Lord by night to ask a question. We are not told directly what his question might have been, but based on the answer given, we can surmise that his question was about The Kingdom and how one enters therein. Every Jew looking forward to Messiah no doubt asked questions along the same lines as The Apostles asked in Acts 1:6. The question was when it would be set up.
The reply is clear and concise. A man (Nicodemus) or thee (Israel, Nicodemus being a representative) must be born again (from above) to see or to enter into The Kingdom. This answers the time as it is when they are resurrected in their new bodies.
So far, up till now, the Lord is the only One who has been born again. Even He could not enter into His Kingdom without being born again or resurrected. When He was Resurrected, He became the firstborn from among the dead and the new head of the human race.
Since He is counted as the firstborn from the dead, then it must follow that all who had been brought to life in the OT and such as The Lord raised in the NT, including Lazarus, the saints raised in Jerusalem at the time of the Lord's Resurrection, and even the various times that Paul died and was made alive; all of these did not have a true resurrection or a rebirth. As late as Col. 1:18, The Lord is still the firstborn from among the dead. And when John wrote his Gospel, The Lord was the only One who had ever ascended to Heaven. See John 3:13. So none other has been born again yet.
Our Lord's first begetting was of The Father, and He was God only begotten. So the Lord was not born of blood, not of the will of the flesh, not of the will of a husband, but of God. He was given to have life in Himself and can give life to as many as it pleases Him.
But what about others? How are they begotten so that they may come to the second birth? We find the answer in 1 Peter 1:3 and 1 Peter 1:23. Peter and other believers of that time had been begotten unto a Living Hope. The Lord's Resurrection accomplished it. In 1 Peter 1:23, it is further explained that they are begotten (not the word for born), by incorruptible seed, by (faith in) The Word of God. So every believer receives this begetting and is thus given everlasting life which comes through faith, and faith cometh by hearing, and hearing by the Word of God (Rom. 10:17).
This begetting is better known as everlasting life, the new nature, the new man, the spirit of God, and other names given. We suggest you read Two Natures in the Child of God by E. W. Bullinger, available for free, and make a list of these names.
Note that this begetting is by incorruptible seed. If that is true, there can be no miscarriage, abortion, or stillbirth, but all must come to a living birth or Resurrection. So, to be born again, one must be a believer, begotten of God of incorruptible seed by The Word.
Nicodemus confessed that he believed, therefore, he was begotten of God. But his question was when the time of The Kingdom would be, and he was told it would be at his rebirth or Resurrection from the dead. The reader is invited to search and see as this Truth will eliminate many difficulties and answer many questions they may have along these lines.
