1 John 3
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 doesn't know us is that it didn't know him.
In everyday language, people say we are all "children of God" and in a sense that's true. Even Apostle Paul uses the term in this sense when preaching at Athens. But strictly speaking, only born-again Christians are "children of God". God Himself has to give the right to a person in order for that person to become His child (John 1:12). It is loosely analogous to an legal adoption process.
2 Beloved, we are God's children now, and what we will be hasn't yet appeared; but we know that when he appears we will 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 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 can't keep on sinning because he has been born of God.
"Born of God" is referring to spiritual regeneration, or to be "born again". The mysterious phenomenon whereby the Holy Spirit, i.e. God's own spirit, enters a person and lives in that person, and they in Him. See also: John 1:13, John 3:3, John 3:5.
10 By this it's evident who are the children of God, and who are the children of the devil: whoever doesn't practice righteousness isn't of God, nor is the one who doesn't love his brother.
Love one another
11 For this is the message that you've heard from the beginning, that we should love one another.
12 We shouldn't be like Cain, who was of the evil one and murdered his brother. And why did he murder him? Because his own deeds were evil and his brother's righteous.
13 Don't be surprised, brothers, that the world hates you.
14 We know that we've passed out of death into life, because we love the brothers. Whoever doesn't love abides in death.
15 Everyone who hates his brother is a murderer, and you know that no murderer has eternal life abiding in him.
16 By this we know love, that he laid down his life for us, and we ought to lay down our lives for the brothers.
17 But if anyone has the world's goods and sees his brother in need, yet closes his heart against him, how does God's love abide in him?
18 Little children, let us not love in word or talk, but in deed and in truth.
19 By this we shall know that we are of the truth and reassure our heart before him;
20 for whenever our heart condemns us, God is greater than our heart, and he knows everything.
21 Beloved, if our heart doesn't condemn us, we have confidence before God;
22 and whatever we ask, we receive from him, because we keep his commandments and do what pleases him.
23 And this is his commandment, that we believe in the name of his Son, Jesus Christ, and love one another, just as he has commanded us.
24 Whoever keeps his commandments abides in him, and he in them. And by this we know that he abides in us, by the Spirit whom he has given us.