Why is Christ only Begotten Son? (John 1:1-18; 3:16; BC 18)

John introduces Jesus as the eternal Word of God, emphasizing His role in creation, bringing light and life to humanity. Jesus takes on the flesh and tabernacles among us. It is God walking in the midst of the human race. However, Jesus does not become the Son of God at that moment, but is Son from eternity. We are assured that as we receive him by faith we are children of God. Yet, there is the language of being born again before we accept Christ. How can Christ be Son of God, and we can also be sons of God by new birth and faith?