The inability of the punishments eternal
From the book: Heaven and Hell – Allan Kardec

Let us imagine a young man of twenty, like so many of today's young men, ignorant, with vicious instincts, denying the existence of his soul and of God, given to uncontrolled behaviour and to committing all sorts of perversities, and then, in a favourable environment, working, learning, gradually correcting himself until he becomes a pious believer. Afterwards, in a favourable environment, this young man works, is instructed, gradually corrects himself until he becomes a pious believer. Is this not a palpable example of the progress of the soul during life, an example which is repeated every day? Such a man dies in old age as a saint, and certainly his salvation is assured. Yet what would have been his fate if an accident had brought him to his death forty or fifty years earlier?
At that time he fulfilled all the conditions necessary for his condemnation; so that, once condemned, every form of progress would be barred to him. We are thus faced with a man who was only saved because he lived longer, and who, according to the doctrine of eternal punishment, would have been lost for ever if he had lived less, perhaps as the result of a fortuitous accident. Since his soul was able to progress at a certain time, why should it not have been able to progress also after death, if a cause beyond his control had prevented him from doing so during his lifetime? Why should God have denied him the means? Repentance, though delayed, would not have failed to come. On the other hand, if from the very moment of his death an irremissible condemnation had been imposed on him, his repentance would have been fruitless for eternity, and his ability to progress would have been annulled for ever.
The dogma of the absolute eternity of punishment is, therefore, incompatible with the progress of souls, to which it opposes an insurmountable barrier. The two principles cancel each other out, for the existence of one necessarily implies the annihilation of the other. Which of the two is real? The law of progress really exists: it is not a theory, but a fact confirmed by experience; it is a law of nature, a divine law, imprescriptible. So, if it exists and cannot be reconciled with the other, then the other does not exist. If the dogma of the eternity of punishment were true, St. Augustine, St. Paul and so many others would never have seen Heaven if they had died before making the progress that led them to conversion.
To this last argument they will reply that the conversion of these saintly characters was not the result of the progress of the soul, but of the grace granted to them and by which they were touched.
However, that is a play on words. If these saints practised evil, and later good, it means that they improved. Why would God have granted them as a special favour the grace to correct themselves? Why to them and not to others? We are always answered with the doctrine of privilege, which is incompatible with God's justice and with the love which He bestows equally on all creatures.
According to spiritism, in accordance with the very words of the Gospel, with logic and with the most rigorous justice, man is the son of his works, both in this life and after death. He owes nothing to grace. God rewards him for the efforts he makes, and punishes him for his negligence as long as he persists in it.