Repentance or tawbah means for one to regret what he/she has done and make a strong resolution and decision to discontinue it; so repenting means for one to if make a commitment between him and Allah (swt) to put an ugly act aside. Kabirah (great) sins entail otherworldly punishment and there is nothing that can compensate for them other than true repentance. Making a commitment and repenting itself is a victory and a window to Allah’s (swt) grace and drives one towards salvation. An individual who reaches this stage (of repentance) has gained a great victory and he must do his utmost to strengthen further his intentions and inclinations towards fighting against sins. In addition, he must attempt to keep himself in this state perpetually. We must pay attention that not all tawbahs are considered true ones and true repentance has necessary conditions which are mentioned below:
1- Committing the sin out of ignorance and foolishness: the Quran says: "[Acceptance of] repentance by Allah is only for those who commit evil out of ignorance."[1] The meaning of ignorance in this verse is broader than the lack of knowledge about a sin and it also covers the situation where one's carnal desires and instincts gain control over his intellect and faith. In this situation, although one does not lose his knowledge in regard to the sin, but his knowledge and faith is overshadowed and immobilized by his desires.  When his knowledge loses its effect, it is as if he has no knowledge at all.
But if the sin was not committed out of ignorance, but rather because of opposing and rejecting the command of Allah, then such a sin reflects one's Kufr (Disbelief) and therefore the repentance of such a person will not be accepted.[2]
2- Not knowing that he will die in the close future (repenting when he still has the chance to commit sins): The repentance of those who are on the verge of death and know it is all over for them has no value. In this regard the Quran says: "But [acceptance of] repentance is not for those who go on committing misdeeds and when death approaches any of them, he says, "I repent now."
3- Avoiding sins: Repenting is meaningless for one who is committing a sin and has still not decided to refrain from it, even if he is determined to repent and avoid the sin in the future, because he may not have the chance to ever repent.[3]
4- Having remorse and regret: If someone is not truly regretful of his sins, he has not truly repented even if he no longer commits the sin, because he may return to it at any moment.
5- Strongly deciding to avoid the sin and not return to it in the future.
6- Compensating for previous sins and shortcomings.
7- Istighfar and asking Allah for forgiveness.[4]
One who truly does tawbah (repentance) with the mentioned conditions should not be hopeless in Allah's mercy, because no matter how great his sin is, Allah's mercy is even greater and Allah will definitely keep his promise and accept his repentance. However, one who prefers to stay impure and insists on his sins definitely still hasn’t regretted committing them, consequently, repenting will be meaningless. Moreover, it is not clear whether or not he will repent in the future and repeating the sin may even take away the tawfiq and chances of repenting! Yes, if one truly repents in a way that its conditions are met, this repentance will be accepted.
One point of dire importance, is the special favor and grace which God bestows upon someone who repents; repentance destroys and effaces all of the sins and negative consequences of sins before God. God has mentioned in the Holy Quran that: ‘Say: "O my Servants who have transgressed against their souls! Despair not of the Mercy of Allah: for Allah forgives all sins: for He is Oft-Forgiving, Most Merciful.[5],[6]
Therefore, you should, as far as possible, try to conceal your sins and not disclose your secrets to anyone. Do not tell anything even to your husband. If he comes to know anything about that, you can honestly let him know about your sincere repentance.
An answer given by Ayatollah Mahdi Hadavi Tehran (may Allah grant him long life) to the above question is as under:
Your remorsefulness and the very decision that you have taken to stay away from such a sinful act cum seeking forgiveness are repentance and they are sufficient. Do not tell anything about it to anyone. If you do not tell it to your husband, you will not be liable to him. In fact, if you tell him, it would be haram.
For further information in this regard, please refer to the following index:
1. Index: Repentance from sins and becoming God’s favorite, 12833.
2. Admitting one’s sin, question 2023 (site: 2078).
3. Index: Knowledge of being forgiven, question 8738 (site: 8741)
4. Index: Ways to become clean from sins, question 470 (site: 511).

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