Objection 1. It seems that to approach this sacrament with consciousness of sin is the gravest of all sins; because the Apostle says (1 Corinthians 11:27): "Whosoever shall eat this bread, or drink the chalice of the Lord unworthily, shall be guilty of the body and of the blood of the Lord": upon which the gloss observes: "He shall be punished as though he slew Christ." But the sin of them who slew Christ seems to have been most grave. Therefore this sin, whereby a man approaches Christ's table with consciousness of sin, appears to be the gravest.
Objection 2. Further, Jerome says in an Epistle (xlix): "What hast thou to do with women, thou that speakest familiarly with God at the altar?" [The remaining part of the quotation is not from St. Jerome]. Say, priest, say, cleric, how dost thou kiss the Son of God with the same lips wherewith thou hast kissed the daughter of a harlot? "Judas, thou betrayest the Son of Man with a kiss!" And thus it appears that the fornicator approaching Christ's table sins as Judas did, whose sin was most grave. But there are many other sins which are graver than fornication, especially the sin of unbelief. Therefore the sin of every sinner approaching Christ's table is the gravest of all.
Objection 3. Further, spiritual uncleanness is more abominable to God than corporeal. But if anyone was to cast Christ's body into mud or a cess-pool, his sin would be reputed a most grave one. Therefore, he sins more deeply by receiving it with sin, which is spiritual uncleanness, upon his soul.
On the contrary, Augustine says on the words, "If I had not come, and had not spoken to them, they would be without sin" (Tract. lxxxix in Joan.), that this is to be understood of the sin of unbelief, "in which all sins are comprised," and so the greatest of all sins appears to be, not this, but rather the sin of unbelief.
I answer that, As stated in I-II, 73, 3,6; II-II, 73, 3, one sin can be said to be graver than another in two ways: first of all essentially, secondly accidentally. Essentially, in regard to its species, which is taken from its object: and so a sin is greater according as that against which it is committed is greater. And since Christ's Godhead is greater than His humanity, and His humanity greater than the sacraments of His humanity, hence it is that those are the gravest sins which are committed against the Godhead, such as unbelief and blasphemy. The second degree of gravity is held by those sins which are committed against His humanity: hence it is written (Matthew 12:32): "Whosoever shall speak a word against the Son of Man, it shall be forgiven him; but he that shall speak against the Holy Ghost, it shall not be forgiven him, neither in this world nor in the world to come." In the third place come sins committed against the sacraments, which belong to Christ's humanity; and after these are the other sins committed against mere creatures.
Accidentally, one sin can be graver than another on the sinner's part. for example, the sin which is the result of ignorance or of weakness is lighter than one arising from contempt, or from sure knowledge; and the same reason holds good of other circumstances. And according to this, the above sin can be graver in some, as happens in them who from actual contempt and with consciousness of sin approach this sacrament: but in others it is less grave; for instance, in those who from fear of their sin being discovered, approach this sacrament with consciousness of sin.
So, then, it is evident that this sin is specifically graver than many others, yet it is not the greatest of all.
Reply to Objection 1. The sin of the unworthy recipient is compared to the sin of them who slew Christ, by way of similitude, because each is committed against Christ's body; but not according to the degree of the crime. Because the sin of Christ's slayers was much graver, first of all, because their sin was against Christ's body in its own species, while this sin is against it under sacramental species; secondly, because their sin came of the intent of injuring Christ, while this does not.
Reply to Objection 2. The sin of the fornicator receiving Christ's body is likened to Judas kissing Christ, as to the resemblance of the sin, because each outrages Christ with the sign of friendship. but not as to the extent of the sin, as was observed above (ad 1). And this resemblance in crime applies no less to other sinners than to fornicators: because by other mortal sins, sinners act against the charity of Christ, of which this sacrament is the sign, and all the more according as their sins are graver. But in a measure the sin of fornication makes one more unfit for receiving this sacrament, because thereby especially the spirit becomes enslaved by the flesh, which is a hindrance to the fervor of love required for this sacrament.
However, the hindrance to charity itself weighs more than the hindrance to its fervor. Hence the sin of unbelief, which fundamentally severs a man from the unity of the Church, simply speaking, makes him to be utterly unfit for receiving this sacrament; because it is the sacrament of the Church's unity, as stated above (Question 61, Article 2). Hence the unbeliever who receives this sacrament sins more grievously than the believer who is in sin; and shows greater contempt towards Christ Who is in the sacrament, especially if he does not believe Christ to be truly in this sacrament; because, so far as lies in him, he lessens the holiness of the sacrament, and the power of Christ acting in it, and this is to despise the sacrament in itself. But the believer who receives the sacrament with consciousness of sin, by receiving it unworthily despises the sacrament, not in itself, but in its use. Hence the Apostle (1 Corinthians 11:29) in assigning the cause of this sin, says, "not discerning the body of the Lord," that is, not distinguishing it from other food: and this is what he does who disbelieves Christ's presence in this sacrament.
Reply to Objection 3. The man who would throw this sacrament into the mire would be guilty of more heinous sin than another approaching the sacrament fully conscious of mortal sin. First of all, because he would intend to outrage the sacrament, whereas the sinner receiving Christ's body unworthily has no such intent; secondly, because the sinner is capable of grace; hence he is more capable of receiving this sacrament than any irrational creature. Hence he would make a most revolting use of this sacrament who would throw it to dogs to eat, or fling it in the mire to be trodden upon.
Source: Summa Theologica (New Advent)