Repentance is righteousness
I commit a crime and so I am a criminal! I'm subjected to a punishment that is judged to be what the crime deserves and it seems to be a fair judgment. While I am enduring the punishment I cherish the crime and regret nothing, so not only have I committed a criminal act I have the heart of a criminal. The heart that expressed itself in opposition to society in a specific act continues to oppose the righteousness to which society pays homage by imprisoning me. I may acknowledge the justice of the laws and the fairness of the punishment but that's as far as it goes—I am a foe of society and its laws which express its conception of social righteousness. (James 2:9-10 has this thought in mind.)
During the course of the imprisonment I have a change of heart. I come to regret my crime and come to see it for what it is—an unjustified act of criminal behaviour. But it isn't just the crime I now denounce; I denounce the criminal heart that expressed itself in that act. The change is a heart and attitude change as well as being a new response to a specific past act.
In this change, I the criminal have crossed over to the ranks of the righteous. I join with the judge and jury in support not only of the punishment I now endure but in support of the reasons and motivations that led them to establish the laws, find me guilty of breaking them and punishing me for it.
The deeper and more passionate my conviction on these matters grows the farther I am removed from the heart of a criminal. I can't alter the fact that I have committed the crime but if I have a thoroughgoing change of heart I am no longer the man who did. I now oppose the man who committed the crime and I oppose the man who for some time cherished it and had no commitment to justice.
The past act can't be undone but the past act can be judged in righteousness and a commitment can be made to a love for and a practice of righteousness from this day forward. As surely as the heart that conceived and effected the crime is a criminal heart, the heart that denounces and renounces the crime is a righteous heart.
The Christian in a biblical and religious setting would speak of "sin" and "repentance" and it would be the case that more than a single sin would be in view.
I commit a grievous sin (or one of the "respectable" sins), I'm judged and in some shape or form I'm punished by the community of believers. I sinned because I had the heart for it and though I'm enduring the inevitable sense of loss I'm bitter enough to feel no remorse much less repentance. If my crime is slander or adultery and I have no remorse or repentance I remain a slanderer or an adulterer (compare Matthew 5:21-23, 27-28). But if I have a change of heart (2 Timothy 2:25) and now in a penitent spirit I acknowledge the truth of God for which the assembly stood, I move from the sinner's camp and become part of the righteous community.
It would be correct to say of me he committed slander or adultery but it would not be correct to now say of me that I am a slanderer or an adulterer. Repentance is the form righteousness takes in someone who has sinned. Jesus, for example, could not personally repent because he had no sin to repent of so his righteousness was always a positive holiness. The rest of us, though we are sinners, are called to be righteous in our living (1 John 2:3-6 and 3:3, for example). We are not to conclude that Jesus holds us as enemies of righteousness because we sin for genuine repentance is righteousness. It is a mind-set, generated in us by the gracious God and in repentance we cross over to his side in the matter of sin (ours or another's).
The deeper and purer the repentance the less we are like the people we were when we sinned without caring. The deeper and purer the repentance the more our heart is like the heart of God. Can you imagine what we will be like as our repentance moves nearer and nearer the ideal?
[These remarks on repentance take into account only one aspect of the multifaceted riches of the biblical witness on repentance but I'm certain they aren't a distortion of that richer truth.]