Read John 3. The Lord Jesus Christ spoke to Nicodemus about being born again:
John 3:16-18 For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life. For God sent not his Son into the world to condemn the world; but that the world through him might be saved. He that believeth on him is not condemned: but he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God.
Paul, speaking to the jailer in Acts 16:
Acts 16:27-32 And the keeper of the prison awaking out of his sleep, and seeing the prison doors open, he drew out his sword, and would have killed himself, supposing that the prisoners had been fled. But Paul cried with a loud voice, saying, Do thyself no harm: for we are all here. Then he called for a light, and sprang in, and came trembling, and fell down before Paul and Silas, And brought them out, and said, Sirs, what must I do to be saved? And they said, Believe on the Lord Jesus Christ, and thou shalt be saved, and thy house. And they spake unto him the word of the Lord, and to all that were in his house.
Romans 1:16 For I am not ashamed of the gospel of Christ: for it [the gospel of Christ] is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek.
a person hears the gospel of Christ.
Because the gospel of Christ is the power of God unto salvation to those who believe, once a person believes the gospel, he or she is born again through the power of God.
The moment a person is born again, he or she is sealed with that Holy Spirit of promise (Eph 1:13).
Ephesians 1:13 In whom [the Lord Jesus Christ] ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise
after you heard the word of truth, the gospel of your salvation, you believed
after you believed, you were sealed with that Holy Spirit of promise (regenerated)
.