Objection 1. It would seem that the fire of the final conflagration is to follow the judgment. For Augustine (De Civ. Dei xx, 30) gives the following order of the things to take place at the judgment, saying: "At this judgment we have learned that the following things will occur. Elias the Thesbite will appear, the Jews will believe, Antichrist will persecute, Christ will judge, the dead shall rise again, the good shall be separated from the wicked, the world shall be set on fire and shall be renewed." Therefore the burning will follow the judgment.
Objection 2. Further, Augustine says (De Civ. Dei xx, 16): "After the wicked have been judged, and cast into everlasting fire, the figure of this world will perish in the furnace of worldly flames." Therefore the same conclusion follows.
Objection 3. Further, when the Lord comes to judgment He will find some men living, as appears from the words of 1 Thessalonians 4:16, where the Apostle speaking in their person says: "Then we who are alive, who remain unto the coming of the Lord [Vulgate: 'who are left, shall be taken... to meet Christ'--the words "who remain," etc., are from 1 Thessalonians 4:14." But it would not be so, if the burning of the world were to come first, since they would be destroyed by the fire. Therefore this fire will follow the judgment.
Objection 4. Further, it is said that our Lord will come to judge the earth by fire, and consequently the final conflagration would seem to be the execution of the sentence of Divine judgment. Now execution follows judgment. Therefore that fire will follow the judgment.
On the contrary, It is written (Psalm 96:3): "A fire shall go before Him."
Further, the resurrection will precede the judgment, else every eye would not see Christ judging. Now the burning of the world will precede the resurrection, for the saints who will rise again will have spiritual and impassible bodies, so that it will be impossible for the fire to cleanse them, and yet the text (Sent. iv, D, 47) quotes Augustine (De Civ. Dei xx, 18) as saying that "whatever needs cleansing in any way shall be cleansed by that fire." Therefore that fire will precede the judgment.
I answer that, The fire in question will in reality, as regards its beginning, precede the judgment. This can clearly be gathered from the fact that the resurrection of the dead will precede the judgment, since according to 1 Thessalonians 4:13-16, those who have slept "shall be taken up... in the clouds... into the air... to meet Christ coming to judgment." Now the general resurrection and the glorification of the bodies of the saints will happen at the same time; for the saints in rising again will assume a glorified body, as evidenced by 1 Corinthians 15:43, "It is sown in dishonor, it shall rise in glory": and at the same time as the saints' bodies shall be glorified, all creatures shall be renewed, each in its own way, as appears from the statement (Romans 8:21) that "the creature... itself shall be delivered from the servitude of corruption into the liberty of the glory of the children of God." Since then the burning of the world is a disposition to the aforesaid renewal, as stated above (1,4); it can clearly be gathered that this burning, so far as it shall cleanse the world, will precede the judgment, but as regards a certain action thereof, whereby it will engulf the wicked, it will follow the judgment.
Reply to Objection 1. Augustine is speaking not as one who decides the point, but as expressing an opinion. This is clear from his continuing thus: "That all these things are to happen is a matter of faith, but how and in what order we shall learn more then by experience of the things themselves than now by seeking a definite conclusion by arguing about them. Methinks, however, they will occur in the order I have given." Hence it is clear that he is speaking as offering his opinion. The same answer applies to the Second Objection.
Reply to Objection 3. All men shall die and rise again: yet those are said to be found alive who will live in the body until the time of the conflagration.
Reply to Objection 4. That fire will not carry out the sentence of the judge except as regards the engulfing of the wicked: in this respect it will follow the judgment.
Source: Summa Theologica (New Advent)