Objection 1. It would seem that it is always a mortal sin to hold communion with an excommunicated person in other cases than those in which it is allowed. Because a certain decretal (Cap. Sacris: De his quae vi, metuve, etc.) declares that "not even through fear of death should anyone hold communion with an excommunicated person, since one ought to die rather than commit a mortal sin." But this would be no reason unless it were always a mortal sin to hold communion with an excommunicated person. Therefore, etc.
Objection 2. Further, it is a mortal sin to act against a commandment of the Church. But the Church forbids anyone to hold communion with an excommunicated person. Therefore it is a mortal sin to hold communion with one who is excommunicated.
Objection 3. Further, no man is debarred from receiving the Eucharist on account of a venial sin. But a man who holds communion with an excommunicated person, outside those cases in which it is allowed, is debarred from receiving the Eucharist, since he incurs a minor excommunication. Therefore it is a mortal sin to hold communion with an excommunicated person, save in those cases in which it is allowed.
Objection 4. Further, no one should incur a major excommunication save for a mortal sin. Now according to the law (Can. Praecipue, seqq., caus. xi) a man may incur a major excommunication for holding communion with an excommunicated person. Therefore it is a mortal sin to hold communion with one who is excommunicated.
On the contrary, None can absolve a man from mortal sin unless he have jurisdiction over him. But any priest can absolve a man for holding communion with those who are excommunicated. Therefore it is not a mortal sin.
Further, the measure of the penalty should be according to the measure of the sin, as stated in Deuteronomy 25:3. Now the punishment appointed by common custom for holding communion with an excommunicated person is not that which is inflicted for mortal sin, but rather that which is due for venial sin. Therefore it is not a mortal sin.
I answer that, Some hold that it is always a mortal sin to hold communion with an excommunicated person, by word or in any of the forbidden ways mentioned above (Article 2), except in those cases allowed by law (Cap. Quoniam). But since it seems very hard that a man should be guilty of a mortal sin by uttering just a slight word to an excommunicated person, and that by excommunicating a person one would endanger the salvation of many, and lay a snare which might turn to one's own hurt, it seems to others more probable that he is not always guilty of a mortal sin, but only when he holds communion with him in a criminal deed, or in an act of Divine worship, or through contempt of the Church.
Reply to Objection 1. This decretal is speaking of holding communion in Divine worship. It may also be replied that the same reason applies both to mortal and venial sin, since just as one cannot do well by committing a mortal sin, so neither can one by committing a venial sin: so that just as it is a man's duty to suffer death rather than commit a mortal sin, so is it his duty to do so sooner than commit a venial sin, inasmuch as it is his duty to avoid venial sin.
Reply to Objection 2. The commandment of the Church regards spiritual matters directly, and legitimate actions as a consequence: hence by holding communion in Divine worship one acts against the commandment, and commits a mortal sin; but by holding communion in other matters, one acts beside the commandment, and sins venially.
Reply to Objection 3. Sometimes a man is debarred from the Eucharist even without his own fault, as in the case of those who are suspended or under an interdict, because these penalties are sometimes inflicted on one person for the sin of another who is thus punished.
Reply to Objection 4. Although it is a venial sin to hold communion with one who is excommunicated, yet to do so obstinately is a mortal sin: and for this reason one may be excommunicated according to the law.
Source: Summa Theologica (New Advent)