1 John 3:1-10 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[a] 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.
Are you a child of God? In this passage, we see what characterizes children of God. The references to “children of…” seem to point to who we follow or the one who leads us. The children of God are those who have been loved by the Father (3:1), who are looking forward to the coming of Christ (3:2) and hope in Him for their purification (3:3). Those who are children of God practice righteousness (3:7 and 3:10) and this is shown by how they love each other (3:10).
On the other hand, those who practice sin and makes a habit of it are not of God. "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" (3:6). "Whoever makes a practice of sinning is of the devil, for the devil has been sinning from the beginning” (3:8). This might seem very harsh but it almost mirrors Jesus’ statement in the book of John: "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 (John 8:44)".
What the Apostle John has been emphasizing from the beginning of this book is that those of us who received the word of life proclaimed by the Apostles (1:1-4), and are in fellowship with God(1:5-10) should live a life that is characterized by keeping God’s commandments(2:1-6) which is summarized by loving each other (2:7-17) and we should stay away from practicing sin but rather pursue righteousness (3:1-10). We do all this by the power of the anointing (2:18-29). The biggest take away for myself is that sin is not something to be trifled with and we should be very wary of making a habit of it. Also we can’t say we are practicing righteousness if we don't consider how we love each other as brothers and sisters in Christ.