1 John 5:1-5 is the clearest, most concise summary of the theology of the entire letter:
[5:1] Everyone who believes that Jesus is the Christ has been born of God, and everyone who loves the Father loves whoever has been born of him.  By this we know that we love the children of God, when we love God and obey his commandments.  For this is the love of God, that we keep his commandments. And his commandments are not burdensome.  For everyone who has been born of God overcomes the world. And this is the victory that has overcome the world—our faith.  Who is it that overcomes the world except the one who believes that Jesus is the Son of God?
John has already written about believing in Jesus as the Christ and the Son of God, loving the brethren, and keeping the commandments, but he does something unique from the rest of his letter here: (1) he shows how these virtues all stem from our being born of God; and (2) he describes these virtues as a great victory over the world.
Everything John mentions here–faith, love, and obedience–he attributes to the miraculous power of the new birth. Note the tenses at work in v. 1: “Everyone who believes (present) that Jesus is the Christ has been born of God (perfect).” If someone believes now, it is because that person has [already] been born of God. As John Stott notes in his commentary, “believing is the consequence, not the cause, of the new birth” (The Epistles of John [Grand Rapids: Eerdmans, 1964], 172).
The same is true concerning our love and obedience to God’s commandments, although you kind of have to read the passage backwards to see how John is saying this. John Piper, in his classic book Desiring God, writes the following concerning 1 John 5:2-4:
Read these sentences in reverse order and notice the logic. First, being born of God gives a power that conquers the world. This is given as the ground or basis (“For”) for the statement that the commandments of God are not burdensome. So being born of God gives a power that conquers our worldly aversion to the will of God. Now His commandments are not “burdensome,” but are the desire and delight of our heart. This is the love of God: not just that we do His commandments, but also that they are not burdensome.
Then in verse 2 the evidence of the genuineness of our love for the children of God is said to be the love of God. What does this teach us about our love for the children of God? Since love for God is doing His will gladly rather than with a sense of burden, and since love for God is the measure of the genuineness of our love for the children of God, therefore our love for the children of God must also be done gladly rather than begrudgingly. (Multnomah, 2003; p. 305)
Our being born of God makes obedience to God’s commandments “not burdensome,” and it creates in us the ability to love the children of God gladly. Not only faith, but love and obedience as well, all come out of our being born of God.
John calls this radical life transformation “the victory that has overcome the world.” The word for victory (and also for the verbs rendered in the ESV as “overcome”) is nike, a word that so captured the essence of “victory” that the Greeks personified that word as the winged goddess Nike, after whom the shoe company named themselves.
“For whoever has been born of God nika the world. And this is the nike that has nikesasa the world–our faith.”
Our victory is the greatest of all victories. Alexander the Great conquered was renowned for his great military victories, but not even he could overcome the whole world. Yet even the humblest, smallest, weakest child who believes upon Jesus has overcome the entire world.
How? By faith. Specifically, by faith that Jesus is the Son of God. Anyone who grasps the significance of this statement does so only because God has caused them to be born again to a new life–a life defined by the person of Jesus Christ, the Son of God.
No one, apart from the work of the Holy Spirit to grant us the new birth, could possibly embrace this thought because it is so foolish to human wisdom. Jesus of Nazareth? Born out of wedlock to a poor teenaged girl? Condemned for blasphemy and cursed by God himself as he died on a tree? This Jesus is the Son of God–the Anointed Christ of God? How could this be?
Only new birth can cause us to overcome the skeptical dogmatism of this world and embrace Jesus wholeheartedly–and only new birth can grant us love for one another and create obedient hearts. Everyone who believes, who loves, and who obeys–that is, everyone who conquers the world–has undoubtedly been born of God.
Post Series on 1 John 5:1-12: