Bottom Line Up Front
Eternal punishment is fitting for the eternally unrepentant.
Quick Answer
This question typically assumes the premise that people are generally good by nature, thereby rendering it inconceivable for a righteous God to condemn a person for merely making a mistake in their rejection of Jesus. However, when you consider the depths of human depravity in light of the holiness of God, the fairness of eternal punishment becomes increasingly evident.1 Even in our sinful nature, we have an inherent desire for justice (which serves as the foundation for this question), demanding retribution from those who wrong us, while vehemently objecting when a situation is unfair. If a child becomes the victim of rape and murder, the family appropriately demands justice, beseeching authorities to hold the perpetrator accountable for his crime. It would be entirely unjust for the magistrate to dismiss the case arbitrarily, releasing the offender without cause. Similarly, the Righteous Judge of all creation would be remiss in letting sin go unpunished. However, God does not want anyone to perish (cf. 2 Peter 3:9), and through His abundant grace and mercy, He executed a plan of redemption through the work of Jesus (cf. John 3:16).2 Therefore, eternal punishment is fitting for the eternally unrepentant, serving as the only equitable punishment for morally debased individuals who relish in their debauchery, while adamantly refusing God’s gracious gift of salvation.