A severe divine declaration seals a powerful prophecy of rebuke, carrying a heavy oath about an irreversible punishment that will follow the sinners to their last day and beyond. The focus is on the extraordinary severity of a sin that leaves no room for ordinary forgiveness.
The primary approach among commentators is that God Himself is speaking, indicating that the words and actions of the people were fully revealed and heard by His ears [אבן עזרא, מצודת דוד, רד״ק, ביאור שטיינזלץ]. Conversely, another view suggests that the prophet is the one speaking, testifying that God's harsh decree was revealed directly to him [אבן עזרא, רד ק בשם התרגום].
The nature of this grave sin is rooted in callousness and apathy [ביאור שטיינזלץ]. It represents a complete lack of faith in God's ability to save them from the impending threat of Sennacherib. Assuming they had no hope of survival, the people sank into deep despair instead of turning to God [שד״ל]. This level of faithlessness is compared to the severe sin of desecrating God's name. For such an offense, standard avenues of atonement like repentance, the Day of Atonement, or personal suffering are simply not enough; only death itself can bring forgiveness [רד״ק, חומת אנך]. Consequently, God swears by His great name that this sin will not be pardoned through exile or any other ordinary means [מלבי״ם, מצודת דוד, רד״ק].
The resulting consequence is that the punishment will last until their dying day. Most commentators explain this literally, noting that the sinners will die by the sword of the besieging enemy, and only this violent end will serve as atonement [אבן עזרא, מצודת דוד, רד״ק]. However, a completely different perspective frames this not as an impending physical execution, but as a permanent spiritual stain. Because the people actually expected to die quickly at the hands of the enemy, the prophet clarifies that they will not perish immediately, as Jerusalem will not fall. Nevertheless, their sin will remain unforgiven forever [שד״ל].
Taking a more spiritual approach, the concept of death here is understood as a second death in the World to Come. The punishment extends far beyond physical mortality, resulting in the death of the soul and the ultimate loss of eternal life [רש״י, רד״ק, חומת אנך]. This implies a continuous, permanent state of punishment [מלבי״ם]. A unique extension of this idea suggests that the sinners will undergo reincarnation, returning to the world only to die a second time in their new life in order to finally rectify their sin [חומת אנך].