Praise be to Allaah.
Firstly: Making mistakes and falling short are undoubtedly part of human nature and no one will be free of shortcomings in his obedience to Allaah, or free of mistakes or forgetfulness or sins. All of us fall short, commit sins, and make mistakes. Sometimes we turn to Allaah and sometimes we turn away from Him. Sometimes we remember that Allaah is watching and sometimes we become negligent. None of us is free of sin, and we will inevitably make mistakes. We are not infallible. Hence the Prophet (peace and blessings of Allaah be upon him) said: By the One in Whose hand is my soul, if you did not commit sin Allaah would do away with you and bring people who would commit sin then pray for forgiveness. Narrated by Muslim, 2749. And he (peace and blessings of Allaah be upon him) said: Every son of Adam sins, and the best of those who sin are those who repent. Narrated by al-Tirmidhi, 2499; classed as hasan by al-Albaani.
By His mercy towards weak man, Allaah has opened the gate of repentance to him, and has commanded him to turn to Him, every time sin overwhelms him. Were it not for that, man would suffer a great deal of hardship and would not have the drive to draw closer to his Lord; he would lose all hope of His forgiveness and pardon. Repentance is something that is necessary as a result of man's shortcomings and human failings.
Allaah has enjoined repentance on all kinds of people in this ummah: those who are foremost in good deeds, those who follow a middle course, and those who wrong their own selves by doing haraam things (cf. Faatir 35:32).
Allaah says (interpretation of the meaning):
And all of you beg Allaah to forgive you all, O believers, that you may be successful
O you who believe! Turn to Allaah with sincere repentance!
And the Messenger of Allaah (peace and blessings of Allaah be upon him) said: O people, repent to Allaah and seek His forgiveness, for I repent one hundred times a day. Narrated by Muslim, 2702 from the hadeeth of al-Agharr al-Muzani (may Allaah be pleased with him).
Allaah has bestowed His mercy and encompassed His slaves with his kindness. He is Forbearing and does not punish us or destroy us straightaway, rather He gives us respite, and He commanded His Prophet (peace and blessings of Allaah be upon him) to proclaim His generosity:
Say: O Ibaadi (My slaves) who have transgressed against themselves (by committing evil deeds and sins)! Despair not of the Mercy of Allaah, verily, Allaah forgives all sins. Truly, He is Oft‑Forgiving, Most Merciful
[al-Zumar 39:53 - interpretation of the meaning].
And He says, out of kindness towards His slaves (interpretation of the meaning):
Will they not turn with repentance to Allaah and ask His forgiveness? For Allaah is Oft‑Forgiving, Most Merciful
And verily, I am indeed forgiving to him who repents, believes (in My Oneness, and associates none in worship with Me) and does righteous good deeds, and then remains constant in doing them (till his death)
And those who, when they have committed Faahishah (illegal sexual intercourse) or wronged themselves with evil, remember Allaah and ask forgiveness for their sins; and none can forgive sins but Allaah and do not persist in what (wrong) they have done, while they know
[Aal Imraan 3:135]
And whoever does evil or wrongs himself but afterwards seeks Allaah's forgiveness, he will find Allaah Oft‑Forgiving, Most Merciful
Allaah calls to repentance those who committed the gravest form of shirk and sin, those who say that Eesa (peace be upon him) is the son of God - exalted be Allaah far above what the wrongdoers say. Allaah says (interpretation of the meaning):
Will they not turn with repentance to Allaah and ask His forgiveness? For Allaah is Oft‑Forgiving, Most Merciful
And He opens the door of repentance to the hypocrites, who are worse than the kuffaar who openly show their kufr. Allaah says (interpretation of the meaning):
Verily, the hypocrites will be in the lowest depth (grade) of the Fire; no helper will you find for them.
146. Except those who repent (from hypocrisy), do righteous good deeds, hold fast to Allaah, and purify their religion for Allaah (by worshipping none but Allaah, and do good for Allaah's sake only, not to show off), then they will be with the believers. And Allaah will grant the believers a great reward
One of the attributes of the Lord is that He accepts repentance and rejoices over it by His grace. Allaah says (interpretation of the meaning):
And He it is Who accepts repentance from His slaves, and forgives sins, and He knows what you do
Know they not that Allaah accepts repentance from His slaves and takes the Sadaqaat (alms, charity), and that Allaah Alone is the One Who forgives and accepts repentance, Most Merciful?
It was narrated that Abu Hamzah Anas ibn Maalik al-Ansaari (may Allaah be pleased with him), the servant of the Messenger of Allaah (peace and blessings of Allaah be upon him), said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: Allaah rejoices more over the repentance of His slave than any one of you who finds his camel after having lost it in a desolate land. Agreed upon.
According to a report narrated by Muslim (2747): Allaah rejoices more over the repentance of His slave than any one of you if he is on his camel in a desolate land, then it runs away from him and on it is his food and drink, and he despairs of finding it, so he goes to a tree and lies down in its shade, having despaired of finding his camel, then while he is like that, it suddenly appears in front of him and he takes hold of its reins and says, because of his intense joy, O Allaah, You are my slave and I am your lord,' making this mistake because of the intensity of his joy.
It was narrated from Abu Moosa Abd-Allaah ibn Qays al-Ash'ari (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: Allaah spreads out His hand at night to accept the repentance of the one who sinned during the day, and He spreads out His hand by day to accept the repentance of the one who sinned during the night, (and that will continue) until the sun rises from the west. Narrated by Muslim, 2759.
It was narrated from Abu Abd al-Rahmaan Abd-Allaah ibn Umar ibn al-Khattaab (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: Allaah will accept the repentance of His slave so long as the death-rattle has not yet reached his throat. Narrated by al-Tirmidhi, 3537; classed as hasan by al-Albaani.
Secondly: The blessings of repentance come in this world and in the Hereafter, some are visible and some are hidden. The rewards of repentance are: purity of heart, erasing of sins and increasing of hasanaat (good deeds). Allaah says (interpretation of the meaning):
O you who believe! Turn to Allaah with sincere repentance! It may be that your Lord will expiate from you your sins, and admit you into Gardens under which rivers flow (Paradise) the Day that Allaah will not disgrace the Prophet (Muhammad) and those who believe with him. Their Light will run forward before them and (with their Records Books of deeds) in their right hands. They will say: Our Lord! Keep perfect our Light for us [and do not put it off till we cross over the Siraat (a slippery bridge over the Hell) safely] and grant us forgiveness. Verily, You are Able to do all things'
The reward of repentance is a good life in the shade of faith, contentment, peace of mind and tranquility. Allaah says (interpretation of the meaning):
Seek the forgiveness of your Lord, and turn to Him in repentance, that He may grant you good enjoyment, for a term appointed, and bestow His abounding Grace to every owner of grace (i.e. the one who helps and serves the needy and deserving, physically and with his wealth, and even with good words)
The reward of repentance is blessings coming down from heaven, blessings coming from the earth, an increase in one's wealth and offspring, blessings in productivity, good physical health and protection from diseases. Allaah says of Hood (peace be upon him) (interpretation of the meaning):
And O my people! Ask forgiveness of your Lord and then repent to Him, He will send you (from the sky) abundant rain, and add strength to your strength, so do not turn away as Mujrimoon (criminals, disbelievers in the Oneness of Allaah)
Thirdly: Everyone who repents to Allaah, Allaah accepts his repentance. The caravan of those who have repented to Allaah will not be interrupted until the sun rises from the west.
One repented from banditry, another from adultery, and others from drinking alcohol, taking drugs, severing the ties of kinship, not praying or being too lazy to pray in congregation, disobeying parents, dealing in riba (usury) and bribes, stealing, shedding blood, consuming people's wealth unlawfully, or smoking. Anyone who repents to Allaah from any sin is to be congratulated, because it is as if they have been born anew through their sincere repentance.
It was narrated from Abu Sa'eed Sa'd ibn Maalik ibn Sinaan al-Khudri (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: There was among the people who came before you a man who killed ninety-nine people. Then he asked about the most knowledgeable person on earth, and was directed to a hermit, so he went to him, told him that he had killed ninety-nine people, and asked if he could be forgiven. The hermit said, No,' so he killed him, thus completing one hundred. Then he asked about the most knowledgeable person on earth and was directed to a scholar. He told him that he had killed one hundred people, and asked whether he could be forgiven. The scholar said, Yes, what could possibly come between you and repentance? Go to such-and-such a town, for in it there are people who worship Allaah. Go and worship with them, and do not go back to your own town, for it is a bad place. So the man set off, but when he was halfway there, the angel of death came to him, and the angels of mercy and the angels of wrath began to argue over him. The angels of mercy said: He had repented and was seeking Allaah.' The angels of wrath said: He never did any good thing.' An angel in human form came to them, and they asked him to decide the matter. He said: Measure the distance between the two lands (his home town and the town he was headed for), and whichever of the two he is closest to is the one to which he belongs.' So they measured the distance, and found that he was closer to the town for which he had been headed, so the angels of mercy took him. (Agreed upon).
According to a version narrated by Muslim (2716): He was closer to the righteous town by a handspan, so he was counted among its people.
According to a version narrated by al-Bukhaari (3470): Allaah commanded (the righteous town) to draw closer and (the evil town) to move away, and he said: Measure the distance between them,' and he was found to be closer to (the righteous town) by a handspan, so he was forgiven.
According to a version narrated by Muslim: He leaned with his chest towards it.
What is meant by repentance is returning to Allaah, giving up sin and hating it, and regretting falling short in obedience to Allaah. Al-Nawawi (may Allaah have mercy on him) said:
Repentance is essential from every sin, even if it is something between a person and Allaah and has nothing to do with the rights of another person. There are three conditions of repentance:
1- You should give up the sin
2- You should regret having done it
3- You should resolve never to go back to it.
If one of these three is missing, then your repentance is not sincere. If the sin has to do with the rights of another person, then there are four conditions: the three mentioned above and restoring the rights of that person. If it is money or property, etc, it must be returned to him; if it had to do with slandering him etc, then you should allow him to insult him in return, or ask for his forgiveness; if it had to do with backbiting about him, then you have to ask for his pardon. It is essential to repent from all sins; if a person repents from some, his repentance from the sins from which he repented is valid - according to the scholars who follow the right path - but he must still repent from the rest as well.
Based on the above, if these conditions are met by the person who is repenting, then there is the hope that his repentance will be accepted, by Allaah's leave. After that he should not worry about waswaas (whispers) suggesting that his repentance is not accepted, because that is from the Shaytaan and is contrary to what Allaah and His Messenger (peace and blessings of Allaah be upon him) have told us, that repentance is accepted if it is sincere.
See also questions no. 624, 13630, 13990, 14289 and 34905.