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. (Colossians 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.
Objection 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.
Objection 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, 74, 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 (Leviticus 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.
I answer that, As stated above (I-II, 72, 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 (Article 1), it disparages the Divine goodness, which is the object of charity. Consequently blasphemy is a mortal sin, by reason of its genus.
Reply to Objection 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 to Objection 2. Since, as stated above (Article 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. (Exodus 20:1). Or else, it is forbidden by the words: "Thou shalt not take the name of... God in vain" (Exodus 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 to Objection 3. There are two ways in which blasphemy may occur unawares and without deliberation. On 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. On 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.
Source: Summa Theologica (New Advent)