I answer that, As stated above (A. 1), blasphemy is opposed to the confession of faith, so that it contains the gravity of unbelief: while the sin is aggravated if the will's detestation is added thereto, and yet more, if it breaks out into words, even as love and confession add to the praise of faith.
Therefore, since, as stated above (Q. 10, A. 3), unbelief is the greatest of sins in respect of its genus, it follows that blasphemy also is a very great sin, through belonging to the same genus as unbelief and being an aggravated form of that sin.
Reply Obj. 1: If we compare murder and blasphemy as regards the objects of those sins, it is clear that blasphemy, which is a sin committed directly against God, is more grave than murder, which is a sin against one's neighbor. On the other hand, if we compare them in respect of the harm wrought by them, murder is the graver sin, for murder does more harm to one's neighbor, than blasphemy does to God. Since, however, the gravity of a sin depends on the intention of the evil will, rather than on the effect of the deed, as was shown above (I-II, Q. 73, A. 8), it follows that, as the blasphemer intends to do harm to God's honor, absolutely speaking, he sins more grievously that the murderer. Nevertheless murder takes precedence, as to punishment, among sins committed against our neighbor.
Reply Obj. 2: A gloss on the words, "Let . . . blasphemy be put away from you" (Eph. 4:31) says: "Blasphemy is worse than perjury." The reason is that the perjurer does not say or think something false about God, as the blasphemer does: but he calls God to witness to a falsehood, not that he deems God a false witness, but in the hope, as it were, that God will not testify to the matter by some evident sign.
Reply Obj. 3: To excuse oneself for sin is a circumstance that aggravates every sin, even blasphemy itself: and it is called the most grievous sin, for as much as it makes every sin more grievous. _______________________
FOURTH ARTICLE [II-II, Q. 13, Art. 4]
Whether the Damned Blaspheme?
Objection 1: It would seem that the damned do not blaspheme. Because some wicked men are deterred from blaspheming now, on account of the fear of future punishment. But the damned are undergoing these punishments, so that they abhor them yet more. Therefore, much more are they restrained from blaspheming.
Obj. 2: Further, since blasphemy is a most grievous sin, it is most demeritorious. Now in the life to come there is no state of meriting or demeriting. Therefore there will be no place for blasphemy.
Obj. 3: Further, it is written (Eccles. 11:3) that "the tree . . . in what place soever it shall fall, there shall it be": whence it clearly follows that, after this life, man acquires neither merit nor sin, which he did not already possess in this life. Now many will be damned who were not blasphemous in this life. Neither, therefore, will they blaspheme in the life to come.