Some have wondered how it is that the penalty of burning in Hell forever can be a just punishment for a single mortal sin. To start, let’s consider the difficulty of salvation. Objectively, salvation is remarkably simple, just be reasonably diligent in knowing your moral obligations, and don’t choose to do anything you know to be grave matter. It is of itself not that hard.
Where the difficulty arises, is in that we as humans have many sinful inclinations, and our wills constantly try to rebel against our intellects. It is thus that we would choose to commit mortal sins, it is not because God made things difficult, it’s because we do.
And it is because we choose of our own free will to reject God’s commands, that we deserve punishment. And when one dies in a state of mortal sin, then one utterly rejects grace, and the possibility of ever repenting.