Objection 1. It would seem that blasphemy is not opposed to the confession of faith. Because to blaspheme is to utter an affront or insult against the Creator. Now this pertains to ill-will against God rather than to unbelief. Therefore blasphemy is not opposed to the confession of faith.
Objection 2. Further, on Ephesians 4:31, "Let blasphemy... be put away from you," a gloss says, "that which is committed against God or the saints."
But confession of faith, seemingly, is not about other things than those pertaining to God, Who is the object of faith. Therefore blasphemy is not always opposed to the confession of faith.
Objection 3. Further, according to some, there are three kinds of blasphemy. The first of these is when something unfitting is affirmed of God; the second is when something fitting is denied of Him; and the third, when something proper to God is ascribed to a creature, so that, seemingly, blasphemy is not only about God, but also about His creatures. Now the object of faith is God. Therefore blasphemy is not opposed to confession of faith.
On the contrary, The Apostle says (1 Timothy 1:12-13): "I... before was a blasphemer and a persecutor," and afterwards, "I did it ignorantly in" my "unbelief." Hence it seems that blasphemy pertains to unbelief.
I answer that, The word blasphemy seems to denote the disparagement of some surpassing goodness, especially that of God. Now God, as Dionysius says (Div. Nom. i), is the very essence of true goodness. Hence whatever befits God, pertains to His goodness, and whatever does not befit Him, is far removed from the perfection of goodness which is His Essence. Consequently whoever either denies anything befitting God, or affirms anything unbefitting Him, disparages the Divine goodness.
Now this may happen in two ways. On 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 is opposed to confession of faith.
Reply to Objection 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 to Objection 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 to Objection 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.
Source: Summa Theologica (New Advent)