One should never despair of the Mercy of Allah because this is a grave major sin. We can see so many verses in the Quran that Allah the Almighty remind us about this grave sin. Here, let’s see some of it.

Allah the Almighty Says (what means):

{And who despairs of the Mercy of his Lord except for those astray?} [Quran 15: 56]

{Indeed, no one despairs of relief from Allah except the disbelieving people.} [Quran 12:87]

{And do not throw [yourselves] with your [own] hands into destruction.} [Quran 2: 195]

Commenting on the verse, some scholars said that when the slave of Allah commits a sin and then thinks that his sin will not be forgiven and, therefore, he abandons seeking forgiveness and repentance to Allah, this is the destruction meant in the verse. We seek refuge in Allah form that.

In a Qudsi Hadeeth (sacred narration), the Prophet narrated that Allah the Almighty Said: “A slave committed a sin and he said, ‘O Allah, Forgive me my sin.’ Allah The Almighty Said, ‘My slave committed a sin and came to realize that he has a Lord Who Forgives sins and Calls to account (the sinner) for the sin.’ He then again committed a sin and said, ‘My Lord, Forgive me my sin.’ Allah, The Exalted, Said, ‘My slave committed a sin and came to realize that he has a Lord Who Forgives sins and Calls to account for the sin.’ He again committed a sin and said, ‘My Lord, Forgive me for my sin.’ Allah, The Exalted, Said, ‘My slave committed a sin and came to realize that he has a Lord Who Forgives sins and Calls to account for the sin. O slave, do what you like. I Have Granted you forgiveness.’” [Muslim]

There is also the example of that man who used to drink alcohol and was brought to the Messenger of Allah . ‘Umar narrated: “During the lifetime of the Prophet there was a man called ‘Abdullah whose nickname was Himar (donkey), and he used to make the Messenger of Allah laugh. The Prophet lashed him because of drinking (alcohol). One day, he was brought to the Prophet for the same charge and was lashed. On that, a man among the people said, ‘O Allah, Curse him! How frequently he is brought (to the Prophet for such a charge)!’ The Prophet said, ‘Do not curse him, for by Allah, I know that he loves Allah and His Messenger.’” [Al-Bukhari]

The wording of another narration reads: A man said: “‘Why does he do so? May Allah Fail him.’ The Messenger of Allah said, ‘Do not support the devil against your brother.’” [Al-Bukhari]

Hence, a slave should never despair of the Mercy of Allah, The Almighty. Rather, whenever he commits a sin, he should ask for forgiveness and repent to Allah. None is infallible. Regarding the righteous people who are promised Paradise, Allah the Almighty Says (what means): {And hasten to forgiveness from your Lord and a garden as wide as the heavens and earth, prepared for the righteous.} [Quran 3: 133]
Indicating the characteristics of the righteous people, Allah Says (what means): {And those who, when they commit an immorality or wrong themselves [by transgression], remember Allah and seek forgiveness for their sins - and who can forgive sins except Allah? - And [who] do not persist in what they have done while they know. Those - their reward is forgiveness from their Lord and gardens beneath which rivers flow [in Paradise], wherein they will abide eternally; and excellent is the reward of the [righteous] workers.} [Quran 3: 135-136]

Even a righteous person may commit a grave major sin or do an immorality. Nevertheless, he immediately gives up the sin and returns to his Lord, seeking forgiveness and repentance.

Here is also the example of the Messengers, may Allah exalt their mention. Allah the Almighty Says (what means): {Indeed, in My presence the messengers do not fear. Otherwise, he who wrongs, then substitutes good after evil - indeed, I Am Forgiving and Merciful.} [Quran 27: 10-11]

Moosa (Moses) who was directly addressed by Allah, killed a person and then said as Allah The Almighty Says (what means): {My Lord, indeed I have wronged myself, so Forgive me,” and He Forgave him. Indeed, He is the Forgiving, the Merciful.} [Quran 28: 16]

Allah the Almighty tells us about the righteous people who made mistakes, Allah Says (what means): {And the one who has brought the truth and [they who] believed in it - those are the righteous. They will have whatever they desire with their Lord. That is the reward of the doers of good. That Allah May Remove from them the worst of what they did and Reward them their due for the best of what they used to do.} [Quran 39: 33-35]

The verses stand as evidence that they did bad actions.