Objection 1. It would seem that Extreme Unction does not avail for the remission of sins. For when a thing can be attained by one means, no other is needed. Now repentance is required in the recipient of Extreme Unction for the remission of his sins. Therefore sins are not remitted by Extreme Unction.
Objection 2. Further, there are no more than three things in sin, the stain, the debt of punishment, and the remnants of sin. Now Extreme Unction does not remit the stain without contrition, and this remits sin even without Unction; nor does it remit the punishment, for if the recipient recover, he is still bound to fulfill the satisfaction enjoined; nor does it take away the remnants of sin, since the dispositions remaining from preceding acts still remain, as may easily be seen after recovery. Therefore remission of sins is by no means the effect of Extreme Unction.
Objection 3. Further, remission of sins takes place, not successively, but instantaneously. On the other hand, Extreme Unction is not done all at once, since several anointings are required. Therefore the remission of sins is not its effect.
On the contrary, It is written (James 5:15): "If he be in sins, they shall be forgiven him."
Further, every sacrament of the New Law confers grace. Now grace effects the forgiveness of sins. Therefore since Extreme Unction is a sacrament of the New Law, its effect is the remission of sins.
I answer that, Each sacrament was instituted for the purpose of one principal effect, though it may, in consequence, produce other effects besides. And since a sacrament causes what it signifies, the principal effect of a sacrament must be gathered from its signification. Now this sacrament is conferred by way of a kind of medicament, even as Baptism is conferred by way of washing, and the purpose of a medicament is to expel sickness. Hence the chief object of the institution of this sacrament is to cure the sickness of sin. Therefore, just as Baptism is a spiritual regeneration, and Penance, a spiritual resurrection, so Extreme Unction is a spiritual healing or cure. Now just as a bodily cure presupposes bodily life in the one who is cured, so does a spiritual cure presuppose spiritual life. Hence this sacrament is not an antidote to those defects which deprive man of spiritual life, namely. original and mortal sin, but is a remedy for such defects as weaken man spiritually, so as to deprive him of perfect vigor for acts of the life of grace or of glory; which defects consist in nothing else but a certain weakness and unfitness, the result in us of actual or original sin. against which weakness man is strengthened by this sacrament. Since, however, this strength is given by grace, which is incompatible with sin, it follows that. in consequence, if it finds any sin, either mortal or venial, it removes it as far as the guilt is concerned, provided there be no obstacle on the part of the recipient; just as we have stated to be the case with regard to the Eucharist and Confirmation (III, 73, 7; III, 79, 3). Hence, too, James speaks of the remission of sin as being conditional, for he says: "If he be in sins, they shall be forgiven him," viz. as to the guilt. Because it does not always blot out sin, since it does not always find any: but it always remits in respect of the aforesaid weakness which some call the remnants of sin. Some, however, maintain that it is instituted chiefly as a remedy for venial sin which cannot be cured perfectly in this lifetime: for which reason the sacrament of the dying is ordained specially against venial sin. But this does not seem to be true, since Penance also blots out venial sins sufficiently during this life as to their guilt, and that we cannot avoid them after doing penance, does not cancel the effect of the previous penance; moreover this is part of the weakness mentioned above.
Consequently we must say that the principal effect of this sacrament is the remission of sin, as to its remnants, and, consequently, even as to its guilt, if it find it.
Reply to Objection 1. Although the principal effect of a sacrament can be obtained without actually receiving that sacrament (either without any sacrament at all, or indirectly by means of some other sacrament), yet it never can be obtained without the purpose of receiving that sacrament. And so, since Penance was instituted chiefly against actual sin, whichever other sacrament may blot out sin indirectly, it does not exclude the necessity of Penance.
Reply to Objection 2. Extreme Unction remits sin in some way as to those three things. For, although the stain of sin is not washed out without contrition, yet this sacrament, by the grace which it bestows, makes the movement of the free will towards sin to be one of contrition, just as may occur in the Eucharist and Confirmation. Again it diminishes the debt of temporal punishment; and this indirectly, in as much as it takes away weakness, for a strong man bears the same punishment more easily than a weak man. Hence it does not follow that the measure of satisfaction is diminished. As to the remnants of sin, they do not mean here those dispositions which result from acts, and are inchoate habits so to speak, but a certain spiritual debility in the mind, which debility being removed, though such like habits or dispositions remain, the mind is not so easily prone to sin.
Reply to Objection 3. When many actions are ordained to one effect, the last is formal with respect to all the others that precede, and acts by virtue of them: wherefore by the last anointing is infused grace which gives the sacrament its effect.
Source: Summa Theologica (New Advent)