From the Supplement, compiled after Aquinas’s death from his Commentary on the Sentences and transcribed here from the 1917 to 1922 printings. About the Supplement.
Text: Summa Theologiae, Supplement to the Third Part (Supplementum Tertiae Partis), transcribed from Internet Archive scans, translated by the Fathers of the English Dominican Province. The Supplement, compiled after Aquinas’s death from his earlier Commentary on the Sentences, in the same English Dominican translation as the Gutenberg Parts. Transcribed by Bibliotheca Catholica from the OCR of four Internet Archive scans of the pre-1931 printings (Questions 1 to 33 from the 1917 Washbourne and Benziger volume, 34 to 68 from the 1922 volume, 69 to 86 from the 1921 volume, and 87 to 99 from the 1922 Benziger volume), with OCR errors corrected and doubtful passages marked, not rewritten. 99 questions and 446 articles. The two appendices printed after Question 99 are not included. Sources, SHA-256 of each OCR file, and the method are in the source record. Rights: Public domain (US)
The Supplement is not a Project Gutenberg text. It is Bibliotheca Catholica’s transcription of public-domain scans at the Internet Archive; obvious OCR errors were corrected, and passages marked “OCR uncertain” could not be confidently corrected, so check them against the linked scan page.
Objection 1: It seems that an indulgence avails those who are in mortal sin. For one person can merit grace and many other good things for another, even though he be in mortal sin. Now indulgences derive their efficacy from the application of the saints' merits to an individual. Therefore they are effective in one who is in mortal sin.
Obj. 2: Further, The greater the need, the more room there is for pity. Now a man who is in mortal sin is in very great need. Therefore all the more should pity be extended to him by indulgence.
On the contrary, A dead member receives no inflow from the other members that are living. But one who is in mortal sin, is like a dead member. Therefore he receives no inflow, through indulgences, from the merits of living members.
I answer that, Some hold that indulgences avail those even who are in mortal sin, for the acquiring of grace, but not for the remission of their punishment, since none can be freed from punishment who is not yet freed from guilt. For he who has not yet been reached by God's operation unto the remission of guilt, cannot receive the remission of his punishment from the minister of the Church, neither by indulgences nor in the tribunal of Penance.
But this opinion seems to be untrue. Because, although those merits which are applied by means of an indulgence, might possibly avail a person so that he could merit grace (by way of congruity and impetration), yet it is not for this reason that they are applied, but for the remission of punishment. Hence they do not avail those who are in mortal sin, and consequently, true contrition and confession are demanded as conditions for gaining all indulgences. If however the merits were applied by such a form as this: / grant you a share in the merits of the whole Churchy--or of one congregation, or of one specified person, then they might avail a person in mortal sin so that he could merit something, as the foregoing opinion holds.
This suffices for the Reply to the First Objection.
Reply Obj. 2: Although he who is in mortal sin is in greater need of help, yet he is less capable of receiving it.
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Also online, linked only: the same translation at New Advent and the Latin text at Corpus Thomisticum. Neither is copied here.