Objection 1. It seems to be lawful to abstain altogether from Communion. Because the Centurion is praised for saying (Matthew 8:8): "Lord, I am not worthy that Thou shouldst enter under my roof"; and he who deems that he ought to refrain entirely from Communion can be compared to the Centurion, as stated above (10, ad 3). Therefore, since we do not read of Christ entering his house, it seems to be lawful for any individual to abstain from Communion his whole life long.
Objection 2. Further, it is lawful for anyone to refrain from what is not of necessity for salvation. But this sacrament is not of necessity for salvation, as was stated above (Question 73, Article 3). Therefore it is permissible to abstain from Communion altogether.
Objection 3. Further, sinners are not bound to go to Communion: hence Pope Fabian (Third Council of Tours, Canon 1) after saying, "Let all communicate thrice each year," adds: "Except those who are hindered by grievous crimes." Consequently, if those who are not in the state of sin are bound to go to Communion, it seems that sinners are better off than good people, which is unfitting. Therefore, it seems lawful even for the godly to refrain from Communion.
On the contrary, Our Lord said (John 6:54): "Except ye eat the flesh of the Son of Man, and drink His blood, you shall not have life in you."
I answer that, As stated above (Article 1), there are two ways of receiving this sacrament namely, spiritually and sacramentally. Now it is clear that all are bound to eat it at least spiritually, because this is to be incorporated in Christ, as was said above (73, 3, ad 1). Now spiritual eating comprises the desire or yearning for receiving this sacrament, as was said above (1, ad 3, 2). Therefore, a man cannot be saved without desiring to receive this sacrament.
Now a desire would be vain except it were fulfilled when opportunity presented itself. Consequently, it is evident that a man is bound to receive this sacrament, not only by virtue of the Church's precept, but also by virtue of the Lord's command (Luke 22:19): "Do this in memory of Me." But by the precept of the Church there are fixed times for fulfilling Christ's command.
Reply to Objection 1. As Gregory says: "He is truly humble, who is not obstinate in rejecting what is commanded for his good." Consequently, humility is not praiseworthy if anyone abstains altogether from Communion against the precept of Christ and the Church. Again the Centurion was not commanded to receive Christ into his house.
Reply to Objection 2. This sacrament is said not to be as necessary as Baptism, with regard to children, who can be saved without the Eucharist, but not without the sacrament of Baptism: both, however, are of necessity with regard to adults.
Reply to Objection 3. Sinners suffer great loss in being kept back from receiving this sacrament, so that they are not better off on that account; and although while continuing in their sins they are not on that account excused from transgressing the precept, nevertheless, as Pope Innocent III says, penitents, "who refrain on the advice of their priest," are excused.
Source: Summa Theologica (New Advent)