8 In the third book the following faulty statements are contained. “If we once admit that, when one vessel is made to honour and another to dishonour, this is due to antecedent causes; why may we not revert to the mystery of the soul and allow that it is loved in one and hated in another because of its past actions, before in Jacob it becomes a supplanter and before in Esau it is supplanted?” And again: “the fact that souls are made some to honour and some to dishonour is to be explained by their previous history.”
And in the same place: “on this hypothesis of mine a vessel made to honour which fails to fulfil its object will in another world become a vessel made to dishonour; and contrariwise a vessel which has from a previous fault been condemned to dishonour will, if it accepts correction in this present life, become in the new creation a vessel 'sanctified and meet for the Master's use and prepared unto every good work.'” And he immediately goes on to say: “I believe that men who begin with small faults may become so hardened in wickedness that, if they do not repent and turn to better things, they must become inhuman energies; and contrariwise that hostile and demonic beings may in course of time so far heal their wounds and check the current of their former sins that they may attain to the abode of the perfect.
As I have often said, in those countless and unceasing worlds in which the soul lives and has its being some grow worse and worse until they reach the lowest depths of degradation; while others in those lowest depths grow better and better until they reach the perfection of virtue.” Thus he tries to show that men, or rather their souls, may become demons; and that demons in turn may be restored to the rank of angels. In the same book he writes: “this too must be considered; why the human soul is diversely acted upon now by influences of one kind and now by influences of another.”
And he surmises that this is due to conduct which has preceded birth. It is for this, he argues, that John leaps in his mother's womb when at Mary's salutation Elizabeth declares herself unworthy of her notice. And he immediately subjoins: “on the other hand infants that are hardly weaned are possessed with evil spirits and become diviners and soothsayers; indeed, some are indwelt from their earliest years with the spirit of a python. Now as they have done nothing to bring upon themselves these visitations, one who holds that nothing happens without God's permission, and that all things are governed by His justice, cannot suppose that God's providence has abandoned them without good reason.”
Source: Letters (New Advent)