Allah, Glorious and Mighty is He, says:
“O you who believe, turn to Allah with sincere repentance so that your Lord may free you from your bad deeds and bring you into Gardens underneath which rivers flow, on the Day when Allah will not disgrace the Prophet and those who believe with him.” (66:8)
For repentance to be true and sincere it must be free from deceit, defects and corruption. A1-Hasan al-Basri said,
"It is when the servant regrets what has happened and resolves never to repeat it again."
"It is when the servant asks for forgiveness with his tongue, feels regret in his heart, and restrains his limbs."
Sa'id ibn al-Musayyib said,
"Sincere repentance is what you purify your souls with."
Ibn al-Qayyim said,
"Being sincere in turning in repentance consists of three things: it must include all the wrong actions of the one who is repenting, leaving none of them aside; it must be accompanied by complete truthfulness and resolve, so that the one who repents does not hesitate or delay, but summons up all his will and determination and embarks upon it wholeheartedly; and it must be free of any impurities and faults that might taint its sincerity, that it is inspired by fear of Allah, hope for what He has, and dread for whatever punishment He might inflict -and not by any desire to safeguard his possessions, or his family, or his social status, or his influence, or to attract people's praise or escape their blame, or to avoid being bothered by nuisances, or to satisfy his appetite for life, or because of his bankruptcy or inability to cope, or any other such ills that would affect the validity of his repentance and his sincerity towards Allah, Mighty and Exalted is He.”
"The first element of sincere repentance concerns the action for which the repentance is made. The second concerns the person who repents himself. The third concerns the One to Whom he repents.”
"The sincerity of the repentance means that it is true and includes all wrong actions. There is no doubt that such turning in repentance requires, and includes, seeking forgiveness, and that it leads to all of the sins that have been committed being wiped out. It is a most excellent and perfect repentance." 4
A servant's sincere turning in repentance to Allah is guaranteed both with forgiveness from Allah even before it takes place, and with forgiveness from Him after it is completed. In other words, the servant turns in repentance between two acts of forgiveness from Allah which secure his salvation. Allah's first act of forgiveness is a permission, an inspiration and a means of assistance which lead to the servant's turning in repentance - which then in turn results in more forgiveness from Allah. The second act of forgiveness is one of acceptance and recompense. Allah, Mighty and Glorious is He, says:
"(He also turned in mercy) to the three who were left behind, when the earth, for all its spaciousness, seemed narrow to them, and their own selves were constricted for them, until they realised that there is no escape from Allah except to Him. Then He turned to them in forgiveness, so that they could turn to Him in repentance; surely Allah is Relenting, Compassionate." (9:118)
Here, Allah, Exalted is He, informs us that His turning to them in forgiveness preceded their turning to Him in repentance, and that it was this that made it possible for them to turn in repentance in the first place. He was the cause of their turning in repentance - which is part of the secret of why He is called 'al-Awwal wa'l-Akhir' - 'the First and the Last'. It is He Who makes things possible and helps to make them happen; the cause is from Him and the consequence is from Him.
The servant is oft-repentant and Allah is oft-Forgiving. The repentance of the servant is his turning back to His Lord after his having turned away. The forgiveness of Allah is of two kinds, one being permission and assistance, and the other being acceptance and reward.
Repentance has a beginning and an end:
Its beginning is turning to Allah by taking the straight path which He has commanded His servants to follow:
And surely this is My straight path so follow it; and do not follow any (other) paths, lest you are separated from His path. (6:153)
Its end is to return to Him on the Day that has already been decreed by taking the path which He has commanded, and which leads to His Garden. Whoever turns in repentance to Allah in this life, Allah will turn to him and reward him at the appointed Time:
“Whoever turns in repentance and does good has truly turned to Allah in true repentance”. (25:71)