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."
But because of our natural unwillingness to repent and believe the glorious gospel, God, in His great mercy, does for sinners what we are unable and unwilling to do for ourselves. He raises us from the death of sin by his grace.- It is what God does for us, not what we do. And this salvation cause us to work and love because salvation is a supernatural work of grace, not an act of fallen, morally impotent, unregenerate will (1 John 3:9; 1 John 5:3-4). Our love for enemies springs only from what Christ did for us, which includes renewing our heart, a healing of our stubborn heart of stone, now made soft as flesh - which gives us a new disposition/inclination to love God and others. The Bible teaches rather that we love because he first loved us and "apart from me you can do nothing"...(John 15:5) . "We know that we have passed out of death into life, because we love the brothers.. because "God's seed abides in us, and we ...have been born of God." (1 John 3:14. 9).
"...and may the Lord cause you to increase and abound in love for one another, and for all people, just as we also do for you;" 1 Thessalonians 3:12