Objection 1. It would seem that this fire will not have such an effect on men as is described in the text (Sent. iv, D, 47). For a thing is said to be consumed when it is reduced to naught. Now the bodies of the wicked will not be reduced to naught, but will be kept for eternity, that they may bear an eternal punishment. Therefore this fire will not consume the wicked, as stated in the text.
Objection 2. Further, if it be said that it will consume the bodies of the wicked by reducing them to ashes; on the contrary, as the bodies of the wicked, so will those of the good be brought to ashes: for it is the privilege of Christ alone that His flesh see not corruption. Therefore it will consume also the good who will then be found.
Objection 3. Further, the stain of sin is more abundant in the elements, as combining together to the formation of the human body wherein is the corruption of the fomes [Cf. I-II, 83, 3; I-II, 91, 6] even in the good, than in the elements existing outside the human body. Now the elements existing outside the human body will be cleansed on account of the stain of sin. Much therefore will the elements in the human body whether of the good or of the wicked need to be cleansed, and consequently the bodies of both will need to be destroyed.
Objection 4. Further, as long as the state of the way lasts the elements act in like manner on the good and the wicked. Now the state of the way will still endure in that conflagration, since after this state of the way death will not be natural, and yet it will be caused by that fire. Therefore that fire will act equally on good and wicked; and consequently it does not seem that any distinction is made between them as to their being affected by that fire, as stated in the text.
Objection 5. Further, this fire will have done its work in a moment as it were. Yet there will be many among the living in whom there will be many things to be cleansed. Therefore that fire will not suffice for their cleansing.
I answer that, This fire of the final conflagration, in so far as it will precede the judgment, will act as the instrument of Divine justice as well as by the natural virtue of fire. Accordingly, as regards its natural virtue, it will act in like manner on the wicked and good who will be alive, by reducing the bodies of both to ashes. But in so far as it acts as the instrument of Divine justice, it will act differently on different people as regards the sense of pain. For the wicked will be tortured by the action of the fire; whereas the good in whom there will be nothing to cleanse will feel no pain at all from the fire, as neither did the children in the fiery furnace (Daniel 3); although their bodies will not be kept whole, as were the bodies of the children: and it will be possible by God's power for their bodies to be destroyed without their suffering pain. But the good, in whom matter for cleansing will be found, will suffer pain from that fire, more or less according to their different merits. On the other hand, as regards the action which this fire will have after the judgment, it will act on the damned alone, since the good will all have impassible bodies.
Reply to Objection 1. Consumption there signifies being brought, not to nothing, but to ashes.
Reply to Objection 2. Although the bodies of the good will be reduced to ashes by the fire, they will not suffer pain thereby, as neither did the children in the Babylonian furnace. In this respect a distinction is drawn between the good and the wicked.
Reply to Objection 3. The elements that are in human bodies, even in the bodies of the elect, will be cleansed by fire. But this will be done, by God's power, without their suffering pain.
Reply to Objection 4. This fire will act not only according to the natural power of the element, but also as the instrument of Divine justice.
Reply to Objection 5. There are three reasons why those who will be found living will be able to be cleansed suddenly. One is because there will be few things in them to be cleansed, since they will be already cleansed by the previous fears and persecutions. The second is because they will suffer pain both while living and of their own will: and pain suffered in this life voluntarily cleanses much more than pain inflicted after death, as in the case of the martyrs, because "if anything needing to be cleansed be found in them, it is cut off by the sickle of suffering," as Augustine says (De Unic. Bap. xiii), although the pain of martyrdom is of short duration in comparison with the pain endured in purgatory. The third is because the heat will gain in intensity what it loses in shortness of time.
Source: Summa Theologica (New Advent)