is there anyone who has ever shown the messengers such obstinate stubbornness (1)?!
(Second) Sin and wrongdoing are a shared human trait; they have always been found among every class of the children of Adam—learned and ignorant, ascetic and worldly, ruler and subject. This is not something unique to this community that could be used to undermine it or its Prophet.
(Third) Sin and disobedience do not negate faith in the messengers. A person may combine within himself Islam and faith on the one hand, and sins and acts of disobedience on the other; the two can coexist. While sins diminish faith’s perfection and completeness, they do not nullify belief in the messengers.
(Fourth) Sins are wiped away by sincere repentance. Even if a person’s offences reached the very clouds of the sky and equalled the number of grains of sand and pebbles, once he repents Allah accepts his repentance. The Exalted says: “Say, ‘O My servants who have transgressed against their own souls, do not despair of Allah’s mercy. Indeed, Allah forgives all sins; He is truly the Ever-Forgiving, the Ever-Merciful.’” ⦗39:53⦘.
This applies to the penitent, for repentance erases whatever came before it, and the one who repents from a sin is like one who never sinned at all. Likewise, tawḥid (monotheism) expiates sins, as stated in the authentic divine hadith (2): “Son of Adam, were you to meet Me with sins that nearly fill the earth, and then meet Me without associating anything with Me, I would meet you with an equal measure of forgiveness.” (3)