Now this may happen in two ways. In the first way it may happen merely in respect of the opinion in the intellect; in the second way this opinion is united to a certain detestation in the affections, even as, on the other hand, faith in God is perfected by love of Him. Accordingly this disparagement of the Divine goodness is either in the intellect alone, or in the affections also. If it is in thought only, it is blasphemy of the heart, whereas if it betrays itself outwardly in speech it is blasphemy of the tongue. It is in this sense that blasphemy is opposed to confession of faith.
Reply Obj. 1: He that speaks against God, with the intention of reviling Him, disparages the Divine goodness, not only in respect of the falsehood in his intellect, but also by reason of the wickedness of his will, whereby he detests and strives to hinder the honor due to God, and this is perfect blasphemy.
Reply Obj. 2: Even as God is praised in His saints, in so far as praise is given to the works which God does in His saints, so does blasphemy against the saints, redound, as a consequence, against God.
Reply Obj. 3: Properly speaking, the sin of blasphemy is not in this way divided into three species: since to affirm unfitting things, or to deny fitting things of God, differ merely as affirmation and negation. For this diversity does not cause distinct species of habits, since the falsehood of affirmations and negations is made known by the same knowledge, and it is the same ignorance which errs in either way, since negatives are proved by affirmatives, according to Poster. i, 25. Again to ascribe to creatures things that are proper to God, seems to amount to the same as affirming something unfitting of Him, since whatever is proper to God is God Himself: and to ascribe to a creature, that which is proper to God, is to assert that God is the same as a creature. _______________________
SECOND ARTICLE [II-II, Q. 13, Art. 2]
Whether Blasphemy Is Always a Mortal Sin?
Objection 1: It would seem that blasphemy is not always a mortal sin. Because a gloss on the words, "Now lay you also all away," etc. (Col. 3:8) says: "After prohibiting greater crimes he forbids lesser sins": and yet among the latter he includes blasphemy. Therefore blasphemy is comprised among the lesser, i.e. venial, sins.
Obj. 2: Further, every mortal sin is opposed to one of the precepts of the decalogue. But, seemingly, blasphemy is not contrary to any of them. Therefore blasphemy is not a mortal sin.
Obj. 3: Further, sins committed without deliberation, are not mortal: hence first movements are not mortal sins, because they precede the deliberation of the reason, as was shown above (I-II, Q. 74, AA. 3, 10). Now blasphemy sometimes occurs without deliberation of the reason. Therefore it is not always a mortal sin.
On the contrary, It is written (Lev. 24:16): "He that blasphemeth the name of the Lord, dying let him die." Now the death punishment is not inflicted except for a mortal sin. Therefore blasphemy is a mortal sin.