4 Bodily substances too are to pass away utterly or else at the end of all things will become highly rarified like the sky and æther and other subtle bodies. It is clear that these principles must affect the writer's view of the resurrection. The sun also and the moon and the rest of the constellations are alive. Nay more; as we men by reason of our sins are enveloped in bodies material and sluggish; so the lights of heaven have for like reasons received bodies more or less luminous, and demons have been for more serious faults clothed with starry frames.
This, he argues, is the view of the apostle who writes:— “the creation has been subjected to vanity and shall be delivered for the revealing of the sons of God.” That it may not be supposed that I am imputing to him ideas of my own I shall give his actual words. “At the end and consummation of the world,” he writes, “when souls and beings endowed with reason shall be released from prison by the Lord, they will move slowly or fly quickly according as they have previously been slothful or energetic.
And as all of them have free will and are free to choose virtue or vice, those who choose the latter will be much worse off than they now are. But those who choose the former will improve their condition. Their movements and decisions in this direction or in that will determine their various futures; whether, that is, angels are to become men or demons, and whether demons are to become men or angels.” Then after adducing various arguments in support of his thesis and maintaining that while not incapable of virtue the devil has yet not chosen to be virtuous, he has finally reasoned with much diffuseness that an angel, a human soul, and a demon— all according to him of one nature but of different wills— may in punishment for great negligence or folly be transformed into brutes.
Moreover, to avoid the agony of punishment and the burning flame the more sensitive may choose to become low organisms, to dwell in water, to assume the shape of this or that animal; so that we have reason to fear a metamorphosis not only into four-footed things but even into fishes. Then, lest he should be held guilty of maintaining with Pythagoras the transmigration of souls, he winds up the wicked reasoning with which he has wounded his reader by saying: “I must not be taken to make dogmas of these things; they are only thrown out as conjectures to show that they are not altogether overlooked.”
Source: Letters (New Advent)