If faith diminishes with sin, does it return with repentance? And will Allah punish the repentant for what their hands have earned, or is there a difference between punishment for a sin and Allah’s testing of His servants? And does punishment expiate the sin itself? And is this exclusive to those who have declared "There is no god but Allah," or does it include all people?

Sincere repentance erases the sins that preceded it, and the one who repents becomes as if he had no sin. He will not be punished for his sin, neither in this world nor in the Hereafter, as Allah the Almighty says: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'" (Quran 39:53). And the Prophet, peace and blessings be upon him, said: "The one who repents from sin is like one who has no sin." As for the calamities that befall a believer in this world, they are an expiation for his misdeeds, as the Prophet, peace and blessings be upon him, said: "No fatigue, nor disease, nor sorrow, nor sadness, nor hurt, nor distress befalls a Muslim, even if it were the prick of a thorn, but Allah expiates some of his sins by it." As for the disbeliever, his sins are not expiated by the afflictions that befall him in this world, because he will abide eternally in Hellfire.