Objection 1. It seems that the forgiveness of mortal sin is an effect of this sacrament. For it is said in one of the Collects (Postcommunion, Pro vivis et defunctis): "May this sacrament be a cleansing from crimes." But mortal sins are called crimes. Therefore mortal sins are blotted out by this sacrament.
Objection 2. Further, this sacrament, like Baptism, works by the power of Christ's Passion. But mortal sins are forgiven by Baptism, as stated above (Question 69, Article 1). Therefore they are forgiven likewise by this sacrament, especially since in the form of this sacrament it is said: "Which shall be shed for many unto the forgiveness of sins."
Objection 3. Further, grace is bestowed through this sacrament, as stated above (Article 1). But by grace a man is justified from mortal sins, according to Romans 3:24: "Being justified freely by His grace." Therefore mortal sins are forgiven by this sacrament.
On the contrary, It is written (1 Corinthians 11:29): "He that eateth and drinketh unworthily, eateth and drinketh judgment to himself": and a gloss of the same passage makes the following commentary: "He eats and drinks unworthily who is in the state of sin, or who handles (the sacrament) irreverently; and such a one eats and drinks judgment, i.e. damnation, unto himself." Therefore, he that is in mortal sin, by taking the sacrament heaps sin upon sin, rather than obtains forgiveness of his sin.
I answer that, The power of this sacrament can be considered in two ways. First of all, in itself: and thus this sacrament has from Christ's Passion the power of forgiving all sins, since the Passion is the fount and cause of the forgiveness of sins.
Secondly, it can be considered in comparison with the recipient of the sacrament, in so far as there is, or is not, found in him an obstacle to receiving the fruit of this sacrament. Now whoever is conscious of mortal sin, has within him an obstacle to receiving the effect of this sacrament; since he is not a proper recipient of this sacrament, both because he is not alive spiritually, and so he ought not to eat the spiritual nourishment, since nourishment is confined to the living; and because he cannot be united with Christ, which is the effect of this sacrament, as long as he retains an attachment towards mortal sin. Consequently, as is said in the book De Eccles. Dogm.: "If the soul leans towards sin, it is burdened rather than purified from partaking of the Eucharist." Hence, in him who is conscious of mortal sin, this sacrament does not cause the forgiveness of sin.
Nevertheless this sacrament can effect the forgiveness of sin in two ways. First of all, by being received, not actually, but in desire; as when a man is first justified from sin. Secondly, when received by one in mortal sin of which he is not conscious, and for which he has no attachment; since possibly he was not sufficiently contrite at first, but by approaching this sacrament devoutly and reverently he obtains the grace of charity, which will perfect his contrition and bring forgiveness of sin.
Reply to Objection 1. We ask that this sacrament may be the "cleansing of crimes," or of those sins of which we are unconscious, according to Psalm 18:13: "Lord, cleanse me from my hidden sins"; or that our contrition may be perfected for the forgiveness of our sins; or that strength be bestowed on us to avoid sin.
Reply to Objection 2. Baptism is spiritual generation, which is a transition from spiritual non-being into spiritual being, and is given by way of ablution. Consequently, in both respects he who is conscious of mortal sin does not improperly approach Baptism. But in this sacrament man receives Christ within himself by way of spiritual nourishment, which is unbecoming to one that lies dead in his sins. Therefore the comparison does not hold good.
Reply to Objection 3. Grace is the sufficient cause of the forgiveness of mortal sin; yet it does not forgive sin except when it is first bestowed on the sinner. But it is not given so in this sacrament. Hence the argument does not prove.
Source: Summa Theologica (New Advent)