I do not believe there is a greater promise in God's Word than that of being born again. It is not used much in the New Testament, and never used by the Apostles in the Book of Acts. It is used today to indicate conversion to Christianity. It is infinitely more than that!
In reply Jesus declared, “I tell you the truth, no one can see the kingdom of God unless he is born again." “How can a man be born when he is old?” Nicodemus asked. “Surely he cannot enter a second time into his mother’s womb to be born!” Jesus answered, “I tell you the truth, no one can enter the kingdom of God unless he is born of water and the Spirit.(B)(John 3:3-5)
For you have been born again, not of perishable seed, but of imperishable, through the living and enduring word of God. (I Peter 1:23)
And what does it have to do with the Kingdom of God?
Being born again is everything, as far as the Royal Priesthood is concerned. It may be true that only the members of the Royal Priesthood are born again; and the people of the nations who are saved to the new world of righteousness, the "sheep," of Matthew 25, are saved by being washed in the blood of the Lamb but are not born again.
I do not like novelties and I don't preach them. This means I think there is something practical and leads toward righteous behavior on the part of God's people, or I would not suggest the being born again is restricted to the Royal Priesthood and distinguishes them from the remainder of the saved people (who are the majority, by the way).