See 1st John 5:4-5
First of all, we are told that whatever is born of God overcomes the world. The world was described as something we are not to love, and is explained in detail in chapter 2, and mentioned again in the fourth chapter in relationship to the spirit of anti-christ. The world is what we were in, but have been taken into the light and life of Jesus Christ, through faith in Jesus Christ.
And, we are said to not only have overcome the world, but, it's explained to us, that "this is the victory that has overcome the world--our faith. Thus faith, or trust in God's Son Jesus Christ as Savior of the World, brings us into this overcoming of the world.
So, faith in Christ is what brings us into victory over the world. Our faith, our trust in, reliance upon, etc, that's the key to the victory.
John, then in typical John fashion, repeats his thought, only with a slight difference. Who is the person who overcomes the world? But, he who believes that Jesus is the Son of God?
It's helpful to remember that John was standing against the many false teachers of his day, who were constantly undermining the truth that the man Jesus of Nazareth was indeed also the Messiah who had existed eternally, and had now come to the earth and taken on a human body.
The man Jesus Christ, is in fact, the Son of God. He is God. He is the Savior of the World. And by coming to Him for salvation, we are changed, born again, redeemed, set apart, justified, adopted into His family made joint heirs with Christ, called saints, pilgrims etc, etc.
Mostly, John's point is that we move from being in the world, to being born of Him. We are no longer citizens of the world, but are now children of God in the family and kingdom of God.
Pastor Bob Grenier
Calvary Chapel Visalia