The Scriptures clearly teach that God’s punishment against sinners lasts forever. The wickedness of sin is aggravated by reason of many things, such as the fact that sin is against a holy, just, gracious, and eternal God.
Revelation 16:9-11, (and verse 21) gives us an additional reason:
Men were scorched with great heat, and blasphemed the name of God, which hath power over these plagues: and they repented not to give him glory. And the fifth angel poured out his vial upon the seat of the beast; and his kingdom was full of darkness; and they gnawed their tongues for pain, and blasphemed the God of heaven because of their pains and their sores, and repented not of their deeds…. And there fell upon men a great hail out of heaven, every stone about the weight of a talent: and men blasphemed God because of the plague of the hail; for the plague thereof was exceeding great.
Judgment does not purge the sinful nature of sinners. Even in the midst of wrath, men do not repent. Instead they shake their fists at God. They continue to blaspheme, the most direct of sins against God. Thus they continue to sin. They continue to be sinners, forever. Therefore, God is just in punishing them forever.
Salvation is by grace alone. The hearts of men are changed by grace alone. The day of grace for sinners is closed when we die, or when Jesus returns. Only the eternal Son of God could take the eternal wrath of God upon himself in our stead.
Thank God we are given eternal life in Jesus. He saves us forever.
Matthew 25:46 And these shall go away into everlasting punishment: but the righteous into life eternal.