Objection 1. It seems that after the resurrection everyone will not be able to know all the sins he has committed. For whatever we know, either we receive it anew through the senses, or we draw it from the treasure house of the memory. Now after the resurrection men will be unable to perceive their sins by means of sense, because they will be things of the past, while sense perceives only the present: and many sins will have escaped the sinner's memory, and he will be unable to recall them from the treasure house of his memory. Therefore after rising again one will not be cognizant of all the sins one has committed.
Objection 2. Further, it is stated in the text (Sent. iv, D, 43), that "there are certain books of the conscience, wherein each one's merits are inscribed." Now one cannot read a thing in a book, unless it be marked down in the book: and sin leaves its mark upon the conscience according to a gloss of Origen on Romans 2:15, "Their conscience bearing witness," etc. which mark, seemingly, is nothing else than the guilt or stain. Since then in many persons the guilt or stain of many sins is blotted out by grace, it would seem that one cannot read in one's conscience all the sins one has committed: and thus the same conclusion follows as before.
Objection 3. Further, the greater the cause the greater the effect. Now the cause which makes us grieve for the sins which we recall to memory is charity. Since then charity is perfect in the saints after the resurrection, they will grieve exceedingly for their sins, if they recall them to memory: yet this is impossible, seeing that according to Apocalypse 21:4, "Sorrow and mourning shall flee away from them." [The quotation is from Isaiah 35:10. The text of the Apocalypse has: "Nor mourning, nor crying, nor sorrow shall be any more."] Therefore they will not recall their own sins to memory.
Objection 4. Further, at the resurrection the damned will be to the good they once did as the blessed to the sins they once committed. Now seemingly the damned after rising again will have no knowledge of the good they once did, since this would alleviate their pain considerably. Neither therefore will the blessed have any knowledge of the sins they had committed.
On the contrary, Augustine says (De Civ. Dei xx) that "a kind of Divine energy will come to our aid, so that we shall recall all of our sins to mind."
Further, as human judgment is to external evidence, so is the Divine judgment to the witness of the conscience, according to 1 Samuel 16:7, "Man seeth those things that appear, but the Lord beholdeth the heart." Now man cannot pass a perfect judgment on a matter unless evidence be taken on all the points that need to be judged. Therefore, since the Divine judgment is most perfect, it is necessary for the conscience to witness to everything that has to be judged. But all works, both good and evil, will have to be judged (2 Corinthians 5:10): "We must all be manifested before the judgment seat of Christ, that every one may receive the proper things of the body, according as he hath done, whether it be good or evil." Therefore each one's conscience must needs retain all the works he has done, whether good or evil.
I answer that, According to Romans 2:15-16, "In the day when God shall judge" each one's conscience will bear witness to him and his thoughts will accuse and defend him. And since in every judicial hearing, the witness, the accuser, and the defendant need to be acquainted with the matter on which judgment has to be pronounced, and since at the general judgment all the works of men will be submitted to judgment, it will behoove every man to be cognizant then of all his works. Wherefore each man's conscience will be as a book containing his deeds on which judgment will be pronounced, even as in the human court of law we make use of records. Of these books it is written in the Apocalypse (20:12): "The books were opened: and another book was opened, which is the book of life; and the dead were judged by those things which were written in the books [Vulgate: 'book'], according to their works." According to Augustine's exposition (De Civ. Dei xx) the books which are here said to be opened "denote the saints of the New and Old Testaments in whom God's commandments are exemplified." Hence Richard of St. Victor (De judic. potest.) says: "Their hearts will be like the code of law." But the book of life, of which the text goes on to speak, signifies each one's conscience, which is said to be one single book, because the one Divine power will cause all to recall their deeds, and this energy, in so far as it reminds a man of his deeds, is called the "book of life" [Cf. I, 24, 1, ad 1]. Or else we may refer the first books to the conscience, and by the second book we may understand the Judge's sentence as expressed in His providence.
Reply to Objection 1. Although many merits and demerits will have escaped our memory, yet there will be none of them but will remain somewhat in its effect, because those merits which are not deadened will remain in the reward accorded to them, while those that are deadened remain in the guilt of ingratitude, which is increased through the fact that a man sinned after receiving grace. In like manner those demerits which are not blotted out by repentance remain in the debt of punishment due to them, while those which have been blotted out by repentance remain in the remembrance of repentance, which they will recall together with their other merits. Hence in each man there will be something whereby he will be able to recollect his deeds. Nevertheless, as Augustine says (De Civ. Dei xx), the Divine energy will especially conduce to this.
Reply to Objection 2. Each one's conscience will bear certain marks of the deeds done by him; and it does not follow that these marks are the guilt alone, as stated above.
Reply to Objection 3. Although charity is now the cause of sorrow for sin, yet the saints in heaven will be so full of joy, that they will have no room for sorrow; and so they will not grieve for their sins, but rather will they rejoice in the Divine mercy, whereby their sins are forgiven them. Even so do the angels rejoice now in the Divine justice whereby those whom they guard fall headlong into sin through being abandoned by grace. and whose salvation none the less they eagerly watch over.
Reply to Objection 4. The wicked will know all the good they have done, and this will not diminish their pain; indeed, it will increase it, because the greatest sorrow is to have lost many goods: for which reason Boethius says (De Consol. ii) that "the greatest misfortune is to have been happy."
Source: Summa Theologica (New Advent)