Some people repent of their sin and are, by definition, righteous.
Others do not repent of their sin but consider themselves righteous anyway. Are they?
CONCERNING REPENTANCE
John 4:
9 Whosoever is begotten of God doeth no sin, because his seed abideth in him: and he cannot sin, because he is begotten of God.
A few observations on this verse.
* The first clause has the term "begotten" in the perfect tense. That greek tense means that the term "begotten" is a past event with present results. The present result is that the person "begotten of God" does not sin (present tense continuous action. In other words, the one who is begotten of God does not practice sin.
* The second clause is even stronger. In the phrase "he cannot sin" the word for "cannot" is dunatai. That verb speaks of ability. The Christian no longer has the ability to live in a state of sin. How then can a Christian repent? The Christian can obviously repent of "sins" but he has already repented of "sin" and does not have the ability to return to "sin." The reason for this present inability to live in "sin" is because of the past action of God in bringing the new birth ("begotten of God")
The Christian has repented in the past by recognizing that he is sinful and a sinner. He recognizes his own rebellion and turns to Christ. The Christian life can be one of repentance of individual sins, such as when David sinned against God with Bathsheba in adultry and murder. But the Christian can never again repent of sin. This is because he can never again live in sin.
CONCERNING RIGHTEOUSNESS
The root word for righteousness and justify is very similar. The meanings must then be very close.
The term justiy has a range of meaning in the scriptures. At times it can mean to be made righteous, other times it can mean to be declared righteous. The context determines the shades of meaning.
One passage that has an obvious context in which the meaning is "declared righteous" is Romans 8
33 Who shall lay anything to the charge of God's elect? It is God that justifieth;
The accused individual comes before the divine judge, he is presented as guilty by the accuser, but the defense lawyer, Jesus Christ appeals that the penalty has been paid (Romans 5:9). So then the divine judge bangs his gavel and pronounces the accused acquitted of all crimes and sins.
So then, is the Christian practically righteous? No, personal sins still occur, but because the Christian does not live in sin, he will always turn away from those personal sins. The Christian is positionally righteous because we were acquitted of all charges at the cross. We are positionally righteous being "in Christ."
CONCLUSIONS
What is righteousness? Righteousness is then being acquitted of all crimes because we are in Christ. But also, the Christian can never live in sin for the same reason, he is in Christ. If one lives in sin, it is evidence that he was never begotten of God. For the one who was never "begotten of God" the question is can he fall? Yes, he can fall into apostacy. In fact I would state it stronger. He will absolutely fall into apostacy. He has no chance to remain in the faith.