Allah has allowed repentance because of unbounded mercy which He has made obligatory on Himself.....

Allah has allowed repentance because of unbounded mercy which He has made obligatory on Himself. "Say: O my servants! who have acted extravagantly against their own souls, do not despair of the mercy of Allah; surely Allah forgives the faults altogether; surely He is the Forgiving, the Merciful." (39:53)

"The Forgiver of the faults and the Acceptor of repentance, severe to punish, Lord of bounty; there is no God but He; to Him is the eventual coming." (40:3)

But the doors of repentance are closed once death approaches: "And repentance is not for those who go on doing evil deeds, until when death comes to one of them, he says 'Surely now I repent', nor (for) those who die while they are unbelievers. These are they for whom We have prepared a painful chastisement." (4:18)

Muhammad ibn Muslim has narrated from Abu Ja'far (peace be upon him) that he said: '"O Muhammad ibn Muslim! Sins of the believer are forgiven to him when he repents from them. Therefore, the believer should perform his deeds afresh after repentance and forgiveness. But, by Allah! It is not but for the people of faith." I said, "But what if he relapses into sins after repentance and forgiveness, and then repents again?" He said, "O Muhammad ibn Muslim! Do you think that a believer servant feels remorse for his sin and asks forgiveness from Allah for it and repents and then Allah will not accept his repentance?" I said, "Then if he does so repeatedly, commits sin and repents and asks forgiveness?" Then he said, "Whenever the believer returns asking for forgiveness and repenting, Allah the High, returns to him with forgiveness; and surely Allah is Forgiving, Merciful. He accepts repentance and pardons the evils. Therefore, be careful, lest you make the believers lose hope of Allah's mercy."' (Al-Kafi)

We conclude our discussion with reciting a part of a du'a, the Whispered Prayer of the Repenters (Taibeen) from Sahifa Sajjadia:

"My God, offenses have clothed me in the garment of my lowliness, separation from Thee has wrapped me in the clothing of my misery! My dreadful crimes have deadened my heart, so bring it to life by a repentance from Thee! O my hope and my aim! O my wish and my want! By Thy might, I find no one but Thee to forgive my sins and I see none but Thee to mend my brokenness! I have subjected myself to Thee in repeated turning, I have humbled myself to Thee in abasement. If Thou castest me out from Thy door, in whom shall I take shelter? If Thou repellest me from Thy side, in whom shall I seek refuge? O my grief at my ignominy and disgrace! O my sorrow at my evil works and what I have committed!"