Despairing of Allah’s mercy leads a person to thinking negatively of his Lord; then his fear increases in severity, and grows increasingly anxious. He may well believe that his sins will never be forgiven, and so he falls into a matter far worse than the sins he had engaged in: [having bad thoughts about Allāh].

The third [major sin mentioned in the ḥadīth] is feeling safe from Allah’s plan [for eventual retribution]—a person feels too safe [about not getting punished], so he downplays the rights of his Lord [upon him], and [thereby] falls into something that must draw the wrath of Allāh, All-Perfect is He and All-High, upon him.

¹Reported by Al-Bazzār, as cited in Majmaʿ Al-Zawāʿid (1/294), and authenticated by Al-Albānī in Ṣaḥīḥ Al-Jāmiʿ (4603).

Source: Al-Najmī, Al-Shaikh Aḥmad ibn Yaḥyá. Al-Sharḥ Al-Mūjaz Al-Mumahhad li Tawḥīd Al-Khāliq Al-Mumajjad, pp. 172-74.

Extract:

Shaikh Aḥmad ibn Yaḥyá Al-Najmī, may Allāh have mercy on him, said:

Despairing of Allah’s mercy [is a major sin that] leads a person to thinking negatively of his Lord; then his fear increases in severity, and he grows increasingly anxious. He may well believe that his sins will never be forgiven, and so he falls into a matter far worse than the sins he had engaged in: [having bad thoughts about Allāh].

[At the same time, it’s also a major sin] to feel safe from Allah’s plan [for a person’s eventual retribution]—a person feels too safe [about not getting punished], so he downplays the rights of his Lord [upon him], and [thereby] falls into something that must draw the wrath of Allāh—All-Perfect is He and All-High—upon him.

Source: Al-Najmī, Al-Shaikh Aḥmad ibn Yaḥyá. Al-Sharḥ Al-Mūjaz Al-Mumahhad li Tawḥīd Al-Khāliq Al-Mumajjad, pp. 172-74.