Tradition suggests the Apostle John was the author of 1 John. There is considerable similarity in the language and thought patterns used in the Gospel of John and 1 John. There is now considerable doubt about authorship, although many scholars think it likely the same author wrote both works during a time of great trial and persecution, sometime around 100 C.E.
The author frequently refers to his audience endearingly as his “children” and uses the idea “brother” (adelphos = interpreted “brother and sister” in the NRSV) to emphasize the type of caring and love that should be extended to others in the worshiping community. In the Gospel of John we find:
Jesus said to them, "The light is with you for a little longer. Walk while you have the light, so that the darkness may not overtake you. If you walk in the darkness, you do not know where you are going. While you have the light, believe in the light, so that you may become children of light." (John 12:35-36) NRSV
In 1 John 2:9-11 we read:
9 Whoever says, "I am in the light," while hating a brother or sister, is still in the darkness. 10 Whoever loves a brother or sister lives in the light, and in such a person there is no cause for stumbling. 11 But whoever hates another believer is in the darkness, walks in the darkness, and does not know the way to go, because the darkness has brought on blindness. NRSV
This idea of “children of light” is a concept common to both works.
The author of 1 John is particularly playing on the Greco-Roman image that children are extensions of their father’s character. Children are of the same essence of their father and children will exhibit a character that is indicative of their origins. Look at how this imagery is used in the following passage to draw the distinction between “children of the light,” those “born of God,” and children of the devil. Then notice how sibling language is used to signify appropriate relationships toward other believers.
1 John 2:29-3:17
2:29 If you know that he is righteous, you may be sure that everyone who does right has been born of him.
3:1 See what love the Father has given us, that we should be called children of God; and that is what we are. The reason the world does not know us is that it did not know him. 2 Beloved, we are God's children now; what we will be has not yet been revealed. What we do know is this: when he is revealed, we will be like him, for we will see him as he is. 3 And all who have this hope in him purify themselves, just as he is pure.
4 Everyone who commits sin is guilty of lawlessness; sin is lawlessness. 5 You know that he was revealed to take away sins, and in him there is no sin. 6 No one who abides in him sins; no one who sins has either seen him or known him. 7 Little children, let no one deceive you. Everyone who does what is right is righteous, just as he is righteous. 8 Everyone who commits sin is a child of the devil; for the devil has been sinning from the beginning. The Son of God was revealed for this purpose, to destroy the works of the devil. 9 Those who have been born of God do not sin, because God's seed abides in them; they cannot sin, because they have been born of God. 10 The children of God and the children of the devil are revealed in this way: all who do not do what is right are not from God, nor are those who do not love their brothers and sisters.
11 For this is the message you have heard from the beginning, that we should love one another. 12 We must not be like Cain who was from 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 Do not be astonished, brothers and sisters, that the world hates you. 14 We know that we have passed from death to life because we love one another. Whoever does not love abides in death. 15 All who hate a brother or sister are murderers, and you know that murderers do not have eternal life abiding in them. 16 We know love by this, that he laid down his life for us -- and we ought to lay down our lives for one another. 17 How does God's love abide in anyone who has the world's goods and sees a brother or sister in need and yet refuses help? NRSV
God has taken us from the family of the devil and made children of God. What is the primary evidence that this has happened? We lovingly treat each other as “brothers,” laying down our lives for each other. The author reiterates this understanding in the next chapter.
1 John 4:20-5:3
20 Those who say, "I love God," and hate their brothers or sisters, are liars; for those who do not love a brother or sister whom they have seen, cannot love God whom they have not seen. 21 The commandment we have from him is this: those who love God must love their brothers and sisters also.
5:1 Everyone who believes that Jesus is the Christ has been born of God, and everyone who loves the parent loves the child. 2 By this we know that we love the children of God, when we love God and obey his commandments. NRSV
Again, at the end of the book.
1 John 5:18-21
18 We know that those who are born of God do not sin, but the one who was born of God protects them, and the evil one does not touch them. 19 We know that we are God's children, and that the whole world lies under the power of the evil one. 20 And we know that the Son of God has come and has given us understanding so that we may know him who is true; and we are in him who is true, in his Son Jesus Christ. He is the true God and eternal life.
While we see this idea of children exhibiting the character of their father in other New Testament works, 1 John employees it as a central theme in helping the Church preserve their identity and faithfulness in a time of great trial.