1 Behold what manner of love the Father has bestowed upon us, that we should be called the sons of God. Therefore, the world does not know us because it did not know him. 2 Beloved, now we are the sons of God, and it does not yet appear what we shall be. But we know that when he appears we shall be like him, for we shall see him as he is. 3 And every man who has this hope in him purifies himself, even as he is pure.
4 Whoever commits sin also transgresses the law, for sin is the transgression of the law. 5 And you know that he was manifested to take away our sins, and in him there is no sin. 6 Whoever abides in him does not sin. Whoever sins has not seen him, neither known him. 7 Little children, let no man deceive you. He who does righteousness is righteous, even as he is righteous. 8 He who commits sin is of the devil, for the devil sins from the beginning. For this purpose the Son of God was manifested so that he might destroy the works of the devil. 9 Whoever is born of God does not commit sin, for his seed remains in him. And he cannot sin because he is born of God. 10 In this the children of God and the children of the devil are manifest: whoever does not do righteousness is not of God, neither he who does not love his brother.
11 For this is the message that you have heard from the beginning: that we should love one another. 12 Not as Cain, who was of that wicked one and slew his brother. And why did he slay him? Because his own works were evil and his brother's were righteous.
13 Do not marvel, my brothers, if the world hates you. 14 We know that we have passed from death to life because we love the brothers. He who does not love his brother abides in death. 15 Whoever hates his brother is a murderer. And you know that no murderer has eternal life abiding in him. 16 In this we know love: because he laid down his life for us. And we ought to lay down our lives for the brothers. 17 But he who has this world's goods and sees his brother have need and shuts up his heart of compassion from him, how does the love of God dwell in him? 18 My little children, let us not love in word, neither in tongue, but in deed and in truth. 19 And by this we know that we are of the truth and shall assure our hearts before him. 20 For if our heart condemns us, God is greater than our heart and knows all things. 21 Beloved, if our heart does not condemn us, then we have confidence toward God. 22 And whatever we ask we receive from him because we keep his commandments and do those things that are pleasing in his sight.
23 And this is his commandment: to believe in the name of his Son Jesus Christ and love one another, as he gave us commandment. 24 And he who keeps his commandments dwells in him, and he in him. And by this we know that he abides in us: by the Spirit which he has given us.
[v.9] - What John is trying to say here is not that those who are born again don't sin, but that they do not obligate themselves to sin. True believers are still human and still have the ability to sin, and often do, but the difference is that there is no planning out and committing to the act of the sin. If a "believer" does commit, or obligate, himself to sin, then he is not born again, and therefore, not a true follower of Christ. Furthermore, when a true believer does sin, he will do so with a pricked conscience and great regret followed by repentance, earnestly seeking God for mercy and forgiveness.
[v.12] - Reference, Genesis 4:8.
[v.15] - Reference, Matthew 5:22.
[v.17] - "the world's goods" - Literally, "the world's means of life."
[v.23] - Reference, Matthew 22:37-40; Mark 12:29-31; John 13:34, 15:12.