Sin is against an infinitely holy God so the punishment is also infinite. But thank the Lord Jesus Christ, in His great love for sinners, came to fully absorb the wrath of God for us upon His own Person; the wrath that sinners like us deserve, so that all who are united to Him will receive His bountiful mercy. This doctrine is perhaps the most sobering in the Bible, but it also teaches us to love our enemies, to do good to those who hate us, to bless those who curse us, and pray for those who abuse us. Because God was kind to ungrateful and evil men like us, so we should likewise be merciful, and extend our love to enemies, even as our Father was merciful to us when we were His enemies. (see Luke 6:27-36).
Regarding eternal punishment Jonathan Edwards once said,
"The crime of one being despising and casting contempt on another, is proportionably more or less heinous, as he was under greater or less obligations to obey him. And therefore if there be any being that we are under infinite obligation to love, and honor, and obey, the contrary towards him must be infinitely faulty. Our obligation to love, honor and obey any being is in proportion to his loveliness, honorableness, and authority. . . . But God is a being infinitely lovely, because he hath infinite excellency and beauty. . . . So sin against God, being a violation of infinite obligations, must be a crime infinitely heinous, and so deserving infinite punishment. . . . The eternity of the punishment of ungodly men renders it infinite . . . and therefore renders it no more than proportionable to the heinousness of what they are guilty of.” (Jonathan Edwards - “The Justice of God in the Damnation of Sinners."