A person who is in the state of grace (habitual grace) is someone who loves God and neighbor. Whatever sinful acts might be on that person’s conscience, he or she is essentially a loving person. The state of grace is the state of loving God and neighbor. Whoever is in a state of grace is a good person. Actual graces help us do good. Habitual grace helps us to be good. The state of grace pertains to who you are: a loving person.
(For an atheist, agnostic, or a believer in some religious or philosophical system that has no discrete concept of God, the state of grace is the state of loving neighbor and thereby implicitly loving God. An atheist can possibly be in the state of grace because, while he does not believe that God exists, he can still believe in loving others selflessly.)
Whosoever dies in a state of grace will have eternal life in Heaven — perhaps after a temporary stay in Purgatory. The place called Purgatory is only temporary because its purpose is to address the sins of persons who are good and loving persons. They died in a state of grace, but with unrepented venial sins and/or unremitted temporal punishment. Purgatory includes just punishment for sin, as well as the change and learning needed to be ready to enter eternal life in Heaven.
Heaven is a place filled to the brim with the perfect love of God and neighbor. Only persons who die in state of grace can possibly go to Heaven. Heaven is a place of love. The state of grace is the state of loving God and neighbor, the state of being a loving person. The souls in Heaven are loving persons in the place of love.
Purgatory is temporary because all the souls there are in the state of grace. They are loving persons and so they cannot remain in a place of punishment. They must eventually go to the place of love.
Hell is a place of just and merciful punishment. But there is a substantial difference between Hell and Purgatory. The souls in Hell are not in the state of grace; they are in a state unrepented actual mortal sin. A person who is in the state of grace loses that state by committing an actual mortal sin; and he remains in a state of mortal sin, until and unless he repents. Every actual mortal sin is entirely incompatible with the love of God and neighbor, and so anyone who is unrepentant from actual mortal sin is not in the state of grace.
All the souls in Hell, without any exception, are condemned to Hell due to unrepented actual mortal sin. No one is sent to Hell for any other reason. And therefore all the souls in Hell are NOT loving persons. They are devoid of true selfless love for God and devoid of true selfless love of neighbor.
Hell is a place of permanent suffering because the souls there are being punished not only for what they did, particular sinful acts, but for what they chose to become by those acts. By choosing actual mortal sin and refusing, through the last moment of life, to repent, they chose to be persons of hate and selfishness, rather than persons of love. They chose to reject the love of God and neighbor, and therefore they must be forever excluded from the place of eternal love: Heaven.
Hell is never-ending because the souls there chose to become something horrible: a human person who has utterly rejected love. Hell is a horrible place because all persons there are devoid of the love of God and neighbor. They lack even an ordered love of self. By choosing actual mortal sin, they chose to reject love. And so they must forever remain in a place devoid of love: Hell. They cannot ever go to Purgatory or Heaven, because both of those places are only for persons who are in the state of loving God and neighbor.
And that is why Hell is permanent, but Purgatory is temporary.
My booklet, Heaven, Hell, Purgatory, and Limbo, is available in Kindle format