I John 3:1-10

1 See what kind of love the Father has given to us, that we should be called children of God; and so we are. The reason why the world does not know us is that it did not know him. 2 Beloved, we are God's children now, and what we will be has not yet appeared; but we know that when he appears we shall be like him, because we shall see him as he is. 3 And everyone who thus hopes in him purifies himself as he is pure.
4 Everyone who makes a practice of sinning also practices lawlessness; sin is lawlessness. 5 You know that he appeared in order to take away sins, and in him there is no sin. 6 No one who abides in him keeps on sinning; no one who keeps on sinning has either seen him or known him. 7 Little children, let no one deceive you. Whoever practices righteousness is righteous, as he is righteous. 8 Whoever makes a practice of sinning is of the devil, for the devil has been sinning from the beginning. The reason the Son of God appeared was to destroy the works of the devil. 9 No one born of God makes a practice of sinning, for God's seed abides in him; and he cannot keep on sinning, because he has been born of God. 10 By this it is evident who are the children of God, and who are the children of the devil: whoever does not practice righteousness is not of God, nor is the one who does not love his brother.

Let's not mistake John to be saying that everyone is called a child of God.  Jesus already had that debate with the Pharisees in John 8:42-27
42 Jesus said to them, “If God were your Father, you would love me, for I came from God and I am here. I came not of my own accord, but he sent me. 43 Why do you not understand what I say? It is because you cannot bear to hear my word. 44 You are of your father the devil, and your will is to do your father's desires. He was a murderer from the beginning, and does not stand in the truth, because there is no truth in him. When he lies, he speaks out of his own character, for he is a liar and the father of lies. 45 But because I tell the truth, you do not believe me. 46 Which one of you convicts me of sin? If I tell the truth, why do you not believe me? 47 Whoever is of God hears the words of God. The reason why you do not hear them is that you are not of God.”

John tells us that if we are abiding in Him, then we should not be surprised when the world treats us the same way it treated Him.  They did not recognize God the Father as the Father of Jesus and they will not recognize God as our Father and Christ as our brother either.  Only those that abide in Christ and are clothed in His righteousness will be able to enter the presence of God.  Remember in Matthew 5 where Jesus told the people that "unless your righteousness exceeds that of the scribes and the Pharisees, you will not enter the kingdom of heaven."  So, again, John is not talking about us and our own righteousness here, but the finished work of Jesus where He takes our unrighteousness and gives us His righteousness in exchange.

Everyone who has been born again must have a new nature, this is taught over and over again and is the key principle that Jesus is trying to teach Nicodemus in John 3:3-7 and in the rest of the chapter as well.
 3 Jesus answered him, “Truly, truly, I say to you, unless one is born again he cannot see the kingdom of God.” 4 Nicodemus said to him, “How can a man be born when he is old? Can he enter a second time into his mother's womb and be born?” 5 Jesus answered, “Truly, truly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God. 6 That which is born of the flesh is flesh, and that which is born of the Spirit is spirit. 7 Do not marvel that I said to you, ‘You must be born again.’

Everyone who says they have received a new life in Christ and claims that they have been changed and have repented and continues to practice sin in such a way where it is obvious to everyone that there has been no change in identity, they are only lying to themselves.  It's a broken analogy because God makes us something completely new and different, but you know when the caterpillar has died and gone through metamorphosis and changed into a butterfly.  Everything about it is made new, even its appetites are different as it no longer desires to eat leaves but drinks the sweet nectar from the flowers.  It is impossible for a caterpillar to go through that change and then go back to being a caterpillar again.  In the same way it is impossible for someone to be born again and continue to live in sin and act like the world.  You are either of the kingdom of light or the kingdom of darkness.....you are a child of God or a child of the devil.  Once your citizenship and identity changes, there is no going back and everything about you is supposed to change, most notably the way that we love God and love each other.