Here, John describes it.
John 1:10-13 He was in the world, and the world was made through him, yet the world did not know him. He came to his own, and his own people did not receive him. But to all who did receive him, who believed in his name, he gave the right to become children of God, who were born, not of blood nor of the will of the flesh nor of the will of man, but of God.
he gave the right to become children of God,
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Well, the way to “become” children of God is precisely why mankind was given the divinely inspired Scriptures. Consequently it is obvious that becoming such children involves a great deal more than the simplistic theology of the Reformation which is usually expressed in one or two sentences!