Objection 1. It seems that the sinner does not sin in receiving Christ's body sacramentally, because Christ has no greater dignity under the sacramental species than under His own. But sinners did not sin when they touched Christ's body under its proper species; nay, rather they obtained forgiveness of their sins, as we read in Luke 7 of the woman who was a sinner; while it is written (Matthew 14:36) that "as many as touched the hem of His garment were healed." Therefore, they do not sin, but rather obtain salvation, by receiving the body of Christ.
Objection 2. Further, this sacrament, like the others, is a spiritual medicine. But medicine is given to the sick for their recovery, according to Matthew 9:12: "They that are in health need not a physician." Now they that are spiritually sick or infirm are sinners. Therefore this sacrament can be received by them without sin.
Objection 3. Further, this sacrament is one of our greatest gifts, since it contains Christ. But according to Augustine (De Lib. Arb. ii), the greatest gifts are those "which no one can abuse." Now no one sins except by abusing something. Therefore no sinner sins by receiving this sacrament.
Objection 4. Further, as this sacrament is perceived by taste and touch, so also is it by sight. Consequently, if the sinner sins by receiving the sacrament, it seems that he would sin by beholding it, which is manifestly untrue, since the Church exposes this sacrament to be seen and adored by all. Therefore the sinner does not sin by eating this sacrament.
Objection 5. Further, it happens sometimes that the sinner is unconscious of his sin. Yet such a one does not seem to sin by receiving the body of Christ, for according to this all who receive it would sin, as exposing themselves to danger, since the Apostle says (1 Corinthians 4:4): "I am not conscious to myself of anything, yet I am not hereby justified." Therefore, the sinner, if he receive this sacrament, does not appear to be guilty of sin.
On the contrary, The Apostle says (1 Corinthians 11:29): "He that eateth and drinketh unworthily, eateth and drinketh judgment to himself." Now the gloss says on this passage: "He eats and drinks unworthily who is in sin, or who handles it irreverently." Therefore, if anyone, while in mortal sin, receives this sacrament, he purchases damnation, by sinning mortally.
I answer that, In this sacrament, as in the others, that which is a sacrament is a sign of the reality of the sacrament. Now there is a twofold reality of this sacrament, as stated above (Question 73, Article 6): one which is signified and contained, namely, Christ Himself; while the other is signified but not contained, namely, Christ's mystical body, which is the fellowship of the saints. Therefore, whoever receives this sacrament, expresses thereby that he is made one with Christ, and incorporated in His members; and this is done by living faith, which no one has who is in mortal sin. And therefore it is manifest that whoever receives this sacrament while in mortal sin, is guilty of lying to this sacrament, and consequently of sacrilege, because he profanes the sacrament: and therefore he sins mortally.
Reply to Objection 1. When Christ appeared under His proper species, He did not give Himself to be touched by men as a sign of spiritual union with Himself, as He gives Himself to be received in this sacrament. And therefore sinners in touching Him under His proper species did not incur the sin of lying to Godlike things, as sinners do in receiving this sacrament.
Furthermore, Christ still bore the likeness of the body of sin; consequently He fittingly allowed Himself to be touched by sinners. But as soon as the body of sin was taken away by the glory of the Resurrection, he forbade the woman to touch Him, for her faith in Him was defective, according to John 20:17: "Do not touch Me, for I am not yet ascended to My Father," i.e. "in your heart," as Augustine explains (Tract. cxxi in Joan.). And therefore sinners, who lack living faith regarding Christ are not allowed to touch this sacrament.
Reply to Objection 2. Every medicine does not suit every stage of sickness; because the tonic given to those who are recovering from fever would be hurtful to them if given while yet in their feverish condition. So likewise Baptism and Penance are as purgative medicines, given to take away the fever of sin; whereas this sacrament is a medicine given to strengthen, and it ought not to be given except to them who are quit of sin.
Reply to Objection 3. By the greatest gifts Augustine understands the soul's virtues, "which no one uses to evil purpose," as though they were principles of evil. Nevertheless sometimes a man makes a bad use of them, as objects of an evil use, as is seen in those who are proud of their virtues. So likewise this sacrament, so far as the sacrament is concerned, is not the principle of an evil use, but the object thereof. Hence Augustine says (Tract. lxii in Joan.): "Many receive Christ's body unworthily; whence we are taught what need there is to beware of receiving a good thing evilly... For behold, of a good thing, received evilly, evil is wrought": just as on the other hand, in the Apostle's case, "good was wrought through evil well received," namely, by bearing patiently the sting of Satan.
Reply to Objection 4. Christ's body is not received by being seen, but only its sacrament, because sight does not penetrate to the substance of Christ's body, but only to the sacramental species, as stated above (Question 76, Article 7). But he who eats, receives not only the sacramental species, but likewise Christ Himself Who is under them. Consequently, no one is forbidden to behold Christ's body, when once he has received Christ's sacrament, namely, Baptism: whereas the non-baptized are not to be allowed even to see this sacrament, as is clear from Dionysius (Eccl. Hier. vii). But only those are to be allowed to share in the eating who are united with Christ not merely sacramentally, but likewise really.
Reply to Objection 5. The fact of a man being unconscious of his sin can come about in two ways. First of all through his own fault, either because through ignorance of the law (which ignorance does not excuse him), he thinks something not to be sinful which is a sin, as for example if one guilty of fornication were to deem simple fornication not to be a mortal sin; or because he neglects to examine his conscience, which is opposed to what the Apostle says (1 Corinthians 11:28): "Let a man prove himself, and so let him eat of that bread, and drink of the chalice." And in this way nevertheless the sinner who receives Christ's body commits sin, although unconscious thereof, because the very ignorance is a sin on his part.
Secondly, it may happen without fault on his part, as, for instance, when he has sorrowed over his sin, but is not sufficiently contrite: and in such a case he does not sin in receiving the body of Christ, because a man cannot know for certain whether he is truly contrite. It suffices, however, if he find in himself the marks of contrition, for instance, if he "grieve over past sins," and "propose to avoid them in the future" [Cf. Rule of Augustine]. But if he be ignorant that what he did was a sinful act, through ignorance of the fact, which excuses, for instance, if a man approach a woman whom he believed to be his wife whereas she was not, he is not to be called a sinner on that account; in the same way if he has utterly forgotten his sin, general contrition suffices for blotting it out, as will be said hereafter (XP, 2, 3, ad 2); hence he is no longer to be called a sinner.
Source: Summa Theologica (New Advent)