Objection 1. It would seem that suffrages performed by sinners do not profit the dead. For, according to John 9:31, "God doth not hear sinners." Now if their prayers were to profit those for whom they pray, they would be heard by God. Therefore the suffrages performed by them do not profit the dead.
Objection 2. Further, Gregory says (Pastoral i, 11) that "when an offensive person is sent to intercede, the wrath of the angered party is provoked to harsher measures." Now every sinner is offensive to God. Therefore God is not inclined to mercy by the suffrages of sinners, and consequently their suffrages are of no avail.
Objection 3. Further, a person's deed would seem to be more fruitful to the doer than to another. But a sinner merits naught for himself by his deeds. Much less, therefore, can he merit for another.
Objection 4. Further, every meritorious work must be a living work, that is to say, informed by charity. Now works done by sinners are dead. Therefore the dead for whom they are done cannot be assisted thereby.
Objection 5.On the contrary, No man can know for certain about another man whether the latter be in a state of sin or of grace. If, therefore, only those suffrages were profitable that are done by those who are in a state of grace, a man could not know of whom to ask suffrages for his dead, and consequently many would be deterred from obtaining suffrages.
Objection 6. Further, according to Augustine (Enchiridion cix), as quoted in the text (Sent. iv, D, 45), the dead are assisted by suffrages according as while living they merited to be assisted after death. Therefore the worth of suffrages is measured according to the disposition of the person for whom they are performed. Therefore it would appear that it differs not whether they be performed by good or by wicked persons.
I answer that, Two things may be considered in the suffrages performed by the wicked. First, the deed done, for instance the sacrifice of the altar. And since our sacraments have their efficacy from themselves independently of the deed of the doer, and are equally efficacious by whomsoever they are performed, in this respect the suffrages of the wicked profit the departed. Secondly, we may consider the deed of the doer, and then we must draw a distinction; because the deed of a sinner who offers suffrage may be considered--in one way in so far as it is his own deed, and thus it can nowise be meritorious either to himself or to another; in another way in so far as it is another's deed, and this happens in two ways. First, when the sinner, offering suffrages, represents the whole Church; for instance a priest when he performs the burial service in church. And since one in whose name or in whose stead a thing is done is understood to do it himself as Dionysius asserts (Coel. Hier. xiii), it follows that the suffrages of that priest, albeit a sinner, profit the departed. Secondly, when he acts as the instrument of another: for the work of the instrument belongs more to the principal agent. Wherefore, although he who acts as the instrument of another be not in a state of merit, his act may be meritorious on account of the principal agent: for instance if a servant being in sin do any work of mercy at the command of his master who has charity. Hence, if a person dying in charity command suffrages to be offered for him, or if some other person having charity prescribe them, those suffrages avail for the departed, even though the persons by whom they are performed be in sin. Nevertheless they would avail more if those persons were in charity, because then those works would be meritorious on two counts.
Reply to Objection 1. The prayer offered by a sinner is sometimes not his but another's, and consequently in this respect is worthy to be heard by God. Nevertheless, God sometimes hears sinners, when, to wit, they ask for something acceptable to God. For God dispenses His goods not only to the righteous but also to sinners (Matthew 5:45), not indeed on account of their merits, but of His loving kindness. Hence a gloss on John 9:31, "God doth not hear sinners," says that "he speaks as one unanointed and as not seeing clearly."
Reply to Objection 2. Although the sinner's prayer is not acceptable in so far as he is offensive, it may be acceptable to God on account of another in whose stead or at whose command he offers the prayer.
Reply to Objection 3. The reason why the sinner who performs these suffrages gains nothing thereby is because he is not capable of profiting by reason of his own indisposition. Nevertheless, as stated above, it may in some way profit another, who is disposed.
Reply to Objection 4. Although the sinner's deed is not living in so far as it is his own, it may be living in so far as it is another's, as stated above.
Since, however, the arguments in the contrary sense would seem to show that it matters not whether one obtain suffrages from good or from evil persons, we must reply to them also.
Reply to Objection 5. Although one cannot know for certain about another whether he be in the state of salvation, one may infer it with probability from what one sees outwardly of a man: for a tree is known by its fruit (Matthew 7:16).
Reply to Objection 6. In order that suffrage avail another, it is requisite that the one for whom it is performed be capable of availing by it: and a man has become capable of this by his own works which he did in his life-time. This is what Augustine means to say. Nevertheless, those works must be such that they can profit him, and this depends not on the person for whom the suffrage is performed, but rather on the one who offers the suffrages whether by performing them or by commanding them.
Source: Summa Theologica (New Advent)