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Excellent question.
The answer is found in the Greek verb tenses. In English, we do not have all the tenses that Greek has. For example, if I came home from playing golf and my wife asked me, "How did you do today?", and I said, "I hit the ball pretty well", how many times did I hit it? Did I hit it once, or numerous times throughout the day? Now, it probably means that I hit it numerous times, but it COULD mean that I only hit it once. If I asked you how you get to work and you say, "I drive", does that mean that you drive regularly or that you are driving today and that is the only time?
In Greek there are different tenses. You have the Aorist, which indicates a "point in time" activity. But you also have the Present tense, which indicates that something is a continuing action. So in my first illustration above, if I said "I hit" in the Present tense, it means that I hit the ball repeatedly throughout the round. However, if I said it in the Aorist tense, it would indicate that I only hit the ball once, or that only once did I hit the ball well. We call it Linear action (something that continues on) vs. Punctiliar action (something that happens at a point in time).
There is also the Perfect tense that occurs in this passage. The perfect is used to show an action that was completed in the past but has ongoing results.
So, let's look at the passage and see which verbs are present, aorist, and perfect: I John 3:4-10
4. Whosoever committeth (present) sin transgresseth (present) also the law: for sin is the transgression of the law.
5. And ye know that he was manifested (aorist) to take away (aorist) our sins; and in him is (present) no sin.
6. Whosoever abideth (present) in him sinneth (present) not: whosoever sinneth (present) hath not seen him (perfect), neither known (perfect) him.
7. Little children, let no man deceive you: he that doeth (present) righteousness is righteous, even as he is righteous.
8. He that committeth (present) sin is of the devil; for the devil sinneth (present) from the beginning. For this purpose the Son of God was manifested (aorist), that he might destroy (aorist) the works of the devil.
9. Whosoever is born (perfect) of God doth not commit (present) sin; for his seed remaineth (present) in him: and he cannot (present) sin, because he is born (perfect) of God.
10. In this the children of God are manifest (present), and the children of the devil: whosoever doeth (present) not righteousness is not of God, neither he that loveth (present) not his brother.
So, the Greek clears this up.
vs 4. A man who is habitually practicing sin is continually transgressing the law.
vs 5. Christ came one time to take away our sins once and for all, and Christ continually has no sin
vs 6. If you have an ongoing relationship with Christ you will not habitually practice sin and those who do habitually practice sin never knew Christ in the past, because that would have had a present result in their lives
vs 7. Those who habitually do righteousness are righteous - they have been born again
vs 8. Those who continually and habitually sin are not saved because it is the devil who continually and habitually sins. Jesus came one time to destroy once and for all the works of Satan
vs 9. If you were born again in the past, which will have ongoing results in your life, you will not continually and habitually commit sin, because the Holy Spirit continually abides in you, so you can not habitually sin because you were born again and that has a lasting effect on your life.
vs 10. So, you can continually tell who is saved and who isn't: those who continue in sin and habitually do not love the brethren are not of God
We all sin. "If we say that we have no sin, we deceive ourselves, and the truth is not in us." (I John 1:8). However, the believer will be convicted of his sin by the indwelling Holy Spirit, and it will not be a habitual lifestyle for him. Those who live unrighteous lifestyles do so because they are not saved. Those who live righteous lifestyles do so because they are saved. It is through this that we can tell who is a child of God and who is still a child of the Devil.