The confession that “Jesus is the Son of God” was not an easy confession, especially among people of Jewish descent of John’s time. It was tantamount to equating a man with God. Thus, it was a strong statement of faith for those who openly confessed it. And that is why John places it here. To believe that Jesus is the Son of God is also to confess the eternal truths of God revealed to the apostles.1
5 Who is it that overcomes the world except the one who believes that Jesus is the Son of God?