I answer that, As stated above (I-II, Q. 72, A. 5), a mortal sin is one whereby a man is severed from the first principle of spiritual life, which principle is the charity of God. Therefore whatever things are contrary to charity, are mortal sins in respect of their genus. Now blasphemy, as to its genus, is opposed to Divine charity, because, as stated above (A. 1), it disparages the Divine goodness, which is the object of charity. Consequently blasphemy is a mortal sin, by reason of its genus.
Reply Obj. 1: This gloss is not to be understood as meaning that all the sins which follow, are mortal, but that whereas all those mentioned previously are more grievous sins, some of those mentioned afterwards are less grievous; and yet among the latter some more grievous sins are included.
Reply Obj. 2: Since, as stated above (A. 1), blasphemy is contrary to the confession of faith, its prohibition is comprised under the prohibition of unbelief, expressed by the words: "I am the Lord thy God," etc. (Ex. 20:1). Or else, it is forbidden by the words: "Thou shalt not take the name of . . . God in vain" (Ex. 20:7). Because he who asserts something false about God, takes His name in vain even more than he who uses the name of God in confirmation of a falsehood.
Reply Obj. 3: There are two ways in which blasphemy may occur unawares and without deliberation. In the first way, by a man failing to advert to the blasphemous nature of his words, and this may happen through his being moved suddenly by passion so as to break out into words suggested by his imagination, without heeding to the meaning of those words: this is a venial sin, and is not a blasphemy properly so called. In the second way, by adverting to the meaning of his words, and to their blasphemous nature: in which case he is not excused from mortal sin, even as neither is he who, in a sudden movement of anger, kills one who is sitting beside him. _______________________
THIRD ARTICLE [II-II, Q. 13, Art. 3]
Whether the Sin of Blasphemy Is the Greatest Sin?
Objection 1: It would seem that the sin of blasphemy is not the greatest sin. For, according to Augustine (Enchiridion xii), a thing is said to be evil because it does harm. Now the sin of murder, since it destroys a man's life, does more harm than the sin of blasphemy, which can do no harm to God. Therefore the sin of murder is more grievous than that of blasphemy.
Obj. 2: Further, a perjurer calls upon God to witness to a falsehood, and thus seems to assert that God is false. But not every blasphemer goes so far as to say that God is false. Therefore perjury is a more grievous sin than blasphemy.
Obj. 3: Further, on Ps. 74:6, "Lift not up your horn on high," a gloss says: "To excuse oneself for sin is the greatest sin of all." Therefore blasphemy is not the greatest sin.
On the contrary, On Isa. 18:2, "To a terrible people," etc. a gloss says: "In comparison with blasphemy, every sin is slight."