children born not of natural descent, nor of human decision or a husband's will, but born of God.
Do a word study. The words "born of God" are in the passive voice, which basically means God is acting and man is passive.
The passive voice represents the subject as being the recipient of the action. E.g., in the sentence, "The boy was hit by the ball," the boy receives the action.
I understand these verses, it is you that does not understand. Verse 12 says that to those who receive and believe on Jesus's name, to them God gives the power or authority to become the sons of God.
Verse 13 is a continuation as shown by the word "which". Now the scripture is explaining the action that God performed in giving them the power to become a son of God. It is a new birth which man cannot possibly perform.
When it says "not of the will of the flesh, or the will of man" it is not referring to the receiving and believing done by the believer, it is referring to God giving them the power or authority to become a son of God, being born again or regenerated.
What did Paul say to the Philipian jailer when he asked what he must do to be saved?
Acts 16:30 And brought them out, and said, Sirs, what must I do to be saved?
31 And they said, Believe on the Lord Jesus Christ, and thou shalt be saved, and thy house.
How could Paul in all sincerety and honesty tell the Philipian jailer to believe if he was not able without the regeneration of the Holy Spirit? But notice Paul does not even mention the Holy Spirit in his answer, only tells him to believe. And why does a regenerate person need to believe on Christ? Is a regenerate person ever lost? Can you be regenerated and spiritually dead at the same time? Was Paul telling a regenerate man to believe to be saved? That's absurd.
And if Calvinism were true, Paul would have had to tell him to do nothing whatsoever, and that if he was one of the fortunate elect God would regenerate him and give him the ability to have faith and believe on Christ. And further, to be 100% honest, Paul would have to tell this man that he cannot be sure if God will ever regenerate him or not.
So Paul, the greatest apostle of them all, did not understand regeneration? He was careless and forgot to mention that a person must be regenerated by the Spirit to believe? Or was he purposely misleading this poor man?
Which is it?