"No one who commits Zina is a believer at the moment when he is committing Zina; no one who steals is a believer at the moment when he is stealing; no one who drinks wine is a believer at the moment when he is drinking it." - And he mentioned a fourth but I (the narrator) have forgotten it - "When he does that the yoke of Islam is shed from his neck, but if he repents, Allah accepts his repentance."
The Book of Cutting off the Hand of the Thief - Sunan an-Nasa'i
This narration, while not from Sunan an-Nasa'i 4872 specifically, addresses the gravity of major sins and their temporary effect on faith.
Scholarly Commentary on the Hadith
This hadith is understood by the majority of Sunni scholars to mean that the complete, perfect quality of faith (al-īmān al-kāmil) leaves a person when they willingly commit a major sin. It does not mean the person becomes a complete disbeliever (kāfir) and exits the fold of Islam entirely, unless the sin itself constitutes disbelief, like mocking the religion.
The phrase "the yoke of Islam is shed from his neck" is a metaphor for the protection and covenant of faith being temporarily broken by the act of disobedience. It is like a servant who rebels against his master; the bond of loyal service is severed at that moment.
Conditions for the Application of the Ruling
This severe description applies only when the sinner commits the act: 1) while acknowledging its prohibition, 2) without any valid excuse (e.g., under coercion), and 3) while being of sound mind. The "moment" is emphasized to show the instantaneous spiritual consequence of the action.
The Door of Repentance is Open
The most crucial part of the hadith is its conclusion: "but if he repents, Allah accepts his repentance." This affirms the mercy of Allah and that no sin is too great for His forgiveness if the repentance is sincere. Sincere repentance (tawbah) requires: ceasing the sin, regretting it, and resolving not to return to it.