Case: A Muslim commits a major sin. One observer immediately pronounces disbelief; another dismisses the seriousness of the deed in the name of faith.
Outward Muslim status, the gravity of a major sin and the distinct ruling of rejecting a foundational belief must not be collapsed. A major sin by itself does not justify universally declaring its perpetrator an unbeliever; neither does faith make the sin harmless.
Sound judgment keeps the prohibition clear, calls for repentance and avoids unsupported definitive verdicts about the individual.