Consider what John the Elder says in these words:
Everyone who commits sin is guilty of lawlessness; sin is lawlessness. You know that he was revealed to take away sins, and in him there is no sin. No one who abides in him sins; no one who sins has either seen him or known him. Little children, let no one deceive you. Everyone who does what is right is righteous, just as he is righteous. 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. 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. 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 (1 John 3.4-10).
These are some difficult words for many of us to swallow, since they seem to place impossibly high standards on the children of God. Who can say they are without sin? Who can say that they perfectly love their brothers and sisters? Who would have the gall to affirm that he doesn't commit sin? And yet if we admit that we actually do sin, and that we do not perfectly love our brothers and sisters, it would seem we disqualify ourselves from being children of God.
I wonder, however, whether we might not understand John's words by reference to the old self vs. new self dialectic which appears in Paul's letters. Paul often makes reference to the old self, who must be put off, and a new self which is found in Christ and in the Christian life. He speaks about this in Eph 4.20-24, Col 3.9-10, and Rom 6.6, where he explicitly mentions the παλαιὸς ἄνθρωπος, the old person or old self. Christian life becomes a radical change of identity and a reorientation of one's conception of self. The baptized Christian becomes clothed with Christ (Gal 3.27), becomes united with Christ in his death (Rom 6.3-5), and thus assumes a new identity as though he were in Christ; he begins to consider himself dead to sin and alive to God in Jesus Christ (Rom 6.11).
Now the Christian makes these new identifications and changes of identity, and yet finds himself nevertheless doing things that are indubitably not Christlike. Sin is a remaining reality in the life of the Christian, as we all know well from personal experience. How then are we supposed to understand its presence? Aren't we in Christ? Haven't we been united with him?
My proposal is that the new self which is born in the heart of the Christian is that which John says cannot sin; the old self is that which sins and finds itself tempted to sin. The life of the Christian is the battle between these two selves. Consider the story of the two wolves: an old man tells a child that in every person there are two wolves, one representing good and the other evil, and they fight against each other every day; the wolf that wins is the one you feed.
Likewise in the present case. The child of God born in you does not sin, and cannot sin, and when you sin he finds himself repulsed and moves you to repent; the child of the devil in you hates righteousness and cannot but sin, and when he finds you doing what is right, he will oppose you with every bit of power available to him. The one with whom you choose to identify, the one whom you choose to strengthen and with whom to occupy yourself -- that is the one that you will find winning at the end of each day.
When John speaks of the child of God who cannot sin, I take it he is referring to the new self -- those new impulses, that new identity of ours which earnestly desires fellowship with God and righteousness. When she speaks of the children of the devil, I take it he is referring to the old self -- those old impulses and way of doing things, that identity which can't stand the things of God at all. The one can't sin; the other can't help but sin. We are to remember that the Son of God came to do away with this latter self.
The proposal I've given presupposes that a single Christian has both a child of God and a child of the devil in him. But how can a child of the devil still be a Christian? John does say that the children of the devil do not love their brothers and sisters (1 John 3.10). Therefore they are a part of the family of God which is here being discussed; after all, John begins this discussion with the affirmation that we are God's children now (3.1-2). Either he understands all of humanity to have been made God's children through Christ's work, in which case Christians are those who understand this truth and conform themselves to it, or else only those who are baptized and believe are children. In any case, children of the devil have brothers and sisters whom they do not love, and so are nevertheless members of the family.
The idea is that the child of God, the person who truly believes and obeys, the one whom we must become, loves and does what is right and does not sin. The child of the devil, the person who disbelieves and disobeys, the one who is in us and yet whom we must kill, does not believe and does not love the brothers and sisters. The child of God comes out in us when we do what is right and love; the child of the devil comes out when we hate and do what is wrong.