Of God and His Creatures2. In this life is the state of merit and demerit: hence the present life is compared to a warfare and to the days of a hired labourer: Man's life is a warfare upon the earth, and his days as those of a day-labourer (Job vii, 1). But when the state of warfare is over, or the labour of a man hired for the day, then reward or punishment is due at once, according as men have acquitted themselves well or ill in the effort: hence it is said: The reward of thy hired labourer shall not rest with thee till morning (Levit. xix, 13).
3. The order of punishment and reward follows that of offence and merit. Now it is only through the soul that merit and demerit appertain to the body: for nothing is meritorious or demeritorious except for being voluntary. Therefore reward and punishment properly pass from the soul to the body, not to the soul for the body's sake. There is no reason therefore why the resumption of bodies should be waited for in the punishing or rewarding of souls: nay, it seems fitting rather that souls, in which fault or merit had a prior place, should have a priority likewise of punishment or reward.
Hereby is refuted the error of sundry Greeks, who say that before the resurrection of their bodies souls neither mount up to heaven nor are plunged into hell.*
But we must observe that there may be some impediment on the part of the good in the way of their souls receiving their final reward in the vision of God immediately upon their departure from the body. To that vision, transcending as it does all natural created capacity, the creature cannot be raised before it is entirely purified: hence it is said that nothing defiled can enter into it (Wisd. vii, 25), and that the polluted shall not pass through it (Isai. xxxv, 8). Now the pollution of the soul is by sin, which is an inordinate union with lower things: from which pollution it is purified in this life by Penance and other Sacraments. Now it happens sometimes that this process of purification is not entirely accomplished in this life; and the offender remains still a debtor with a debt of punishment upon him, owing to some negligence, or distraction, or to death overtaking him before his debt is paid. Not for this does he deserve to be entirely shut out from reward: because all this may happen without mortal sin; and it is only mortal sin that occasions the loss of charity, to which the reward of life everlasting is due. Such persons then must be cleansed in the next life, before entering upon their eternal reward. This cleansing is done by penal inflictions, as even in this life it might have been completed by penal works of satisfaction: otherwise the negligent would be better off than the careful, if the penalty that men do not pay here for their sins is not to be undergone by them in the life to come. The souls then of the good, who have upon them in this world something that needs cleansing, are kept back from their reward, while they endure cleansing purgatorial pains. And this is the reason why we posit a purgatory, or place of cleansing.
4.90 : How Incorporeal Subsistent Spirits suffer from Corporeal Fire, and are befittingly punished with Corporeal Punishments
4.92 : That the Souls of the Saints after Death have their Will immutably fixed on Good