On the contrary, It is written (1 Cor. 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 Obj. 1: We ask that this sacrament may be the "cleansing of crimes," or of those sins of which we are unconscious, according to Ps. 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 Obj. 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 Obj. 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. _______________________
FOURTH ARTICLE [III, Q. 79, Art. 4]
Whether Venial Sins Are Forgiven Through This Sacrament?
Objection 1: It seems that venial sins are not forgiven by this sacrament, because this is the "sacrament of charity," as Augustine says (Tract. xxvi in Joan.). But venial sins are not contrary to charity, as was shown in the Second Part (I-II, Q. 88, AA. 1, 2; II-II, Q. 24, A. 10). Therefore, since contrary is taken away by its contrary, it seems that venial sins are not forgiven by this sacrament.