These types of questions come up quite often. Essentially, the questions are based on a comparison between what Catholic salvation theology teaches versus what seems fair. And the answer, typically, is that it does seem unfair, and would be unfair, if not for other considerations, which have not been taken into account.
An adult Christian has lived a good life, prayed often, done many works of mercy towards persons in need, loved God and neighbor. Then he commits an actual mortal sin and dies an hour later. Having died unrepentant, he is sent to Hell. That seems unfair.
Additional considerations: The person who lives a holy prayerful life, and who chooses to commit an actual mortal sin, has a greater guilt because living a holy life gives us a greater awareness of what is sinful and how evil sin it. But then, too, the holy person who lives a holy life is able to repent much more easily than the hardened sinner. Therefore, what is most likely, is that the holy person who sins gravely will repent very soon after that grave sin, and will have much help from providence and grace to be able to repent before he dies.
A young Catholic Christian commits an act he knows from Church teaching to be a mortal sin. He dies soon after and goes to Hell. He is young and made one mistake, yet he is punished eternally. That seems unfair.
Additional considerations: The mere knowledge that an act is a mortal sin under Church teaching is not sufficient to make that act an actual mortal sin. Often an objective mortal sin is not also an actual mortal sin due to the many factors that decrease culpability, including youth. So the young person in the example does not go to Hell, but to Purgatory and then Heaven.
A person lives a long and sinful life, with many unrepentant actual mortal sins of great wickedness. Then he repents at the hour of death and goes to Heaven. That seems unfair.
Additional considerations: The more sins a person commits without repentance, and the greater the sins, and the longer the person persists in sin, the less likely they are to repent, even at the hour of death with the thought of Hell before them. Most persons described in this example do not repent.
A Saint can repent of a lifetime of occasional venial sin in an instant. But a person who has lived a long and very sinful life usually requires a significant amount of time to repent. They typically can’t repent in a single moment, and then go to Heaven. They have to consider all the grave sins of their life, and then make a good Confession, OR be repentant with perfect contrition (out of love for God and neighbor).
But if a person does repent, should we not welcome them into eternity? Even so, such a person, dying repentant, will be sent to Purgatory for a very long and very painful punishment. Then, in heaven, that person will have much less glory than the lifelong faithful Christians. The hardened sinner does not escape justice by repenting.
An atheist goes to Heaven, by invincible ignorance and the love of neighbor. A Catholic who lives according to Church teaching also goes to Heaven, but only by living a very strict life in accord with Catholic teaching. It seems unfair that the atheist gets to Heaven with a life that is so much less rigorous, as to what is required of him. And the same could be said for non-Christian believers or even non-Catholic Christians.
Additional considerations: The atheist’s path to Heaven is a dark and meandering path, full of ignorance of truth and goodness, full of unnecessarily sufferings in mind, heart, and soul. The atheist is much less likely to get to Heaven, and if he does obtain Heaven, he has much less glory than the faithful Catholic. The Catholic has a well-lit, level and straight path to Heaven, with many additional helps, greater grace and providence, and an easier path to return to the state of grace if he sins gravely. And the Catholic’s glory in Heaven is much greater.
The path to Heaven for Catholics seems much more dangerous, since Catholics realize that many acts are grave sins, while most other persons, including Protestants, do not have the same fullness of understanding of the moral law. So they often get to Heaven by ignorance, while some Catholics end up in Hell because they had that understanding.
Additional considerations: The considerations that reduce culpability also apply to Catholic sinners. And it is easier for a Catholic to return to the state of grace because we have the Sacrament of Reconciliation. Also, our understanding of morality makes it easier for us to avoid mortal sin, and thereby avoid the sufferings that result from mortal sin. So we are not worse off for having a better understanding of morality.
If you have other situations to propose, add them in comments.
Please take a look at this list of my books and booklets, and see if any topic interests you.