Today’s Catechism sections discuss the second commandment. Supporting material comes from the “Summa Theologica”.

Article 2

THE SECOND COMMANDMENT

You shall not take the name of the Lord your God in vain.72

You have heard that it was said to the men of old, "You shall not swear falsely. . But I say to you, Do not swear at all.73

I. The Name of the Lord is Holy

2142 The second commandment prescribes respect for the Lord's name. Like the first commandment, it belongs to the virtue of religion and more particularly it governs our use of speech in sacred matters.

2143 Among all the words of Revelation, there is one which is unique: the revealed name of God. God confides his name to those who believe in him; he reveals himself to them in his personal mystery. The gift of a name belongs to the order of trust and intimacy. "The Lord's name is holy." For this reason man must not abuse it. He must keep it in mind in silent, loving adoration. He will not introduce it into his own speech except to bless, praise, and glorify it.74

2144 Respect for his name is an expression of the respect owed to the mystery of God himself and to the whole sacred reality it evokes. The sense of the sacred is part of the virtue of religion:

Are these feelings of fear and awe Christian feelings or not? . . . I say this, then, which I think no one can reasonably dispute. They are the class of feelings we should have - yes, have to an intense degree - if we literally had the sight of Almighty God; therefore they are the class of feelings which we shall have, if we realize His presence. In proportion as we believe that He is present, we shall have them; and not to have them, is not to realize, not to believe that He is present.75

2145 The faithful should bear witness to the Lord's name by confessing the faith without giving way to fear.76 Preaching and catechizing should be permeated with adoration and respect for the name of our Lord Jesus Christ.

2146 The second commandment forbids the abuse of God's name, i.e., every improper use of the names of God, Jesus Christ, but also of the Virgin Mary and all the saints.

2147 Promises made to others in God's name engage the divine honor, fidelity, truthfulness, and authority. They must be respected in justice. To be unfaithful to them is to misuse God's name and in some way to make God out to be a liar.77

2148 Blasphemy is directly opposed to the second commandment. It consists in uttering against God - inwardly or outwardly - words of hatred, reproach, or defiance; in speaking ill of God; in failing in respect toward him in one's speech; in misusing God's name. St. James condemns those "who blaspheme that honorable name [of Jesus] by which you are called."78 The prohibition of blasphemy extends to language against Christ's Church, the saints, and sacred things. It is also blasphemous to make use of God's name to cover up criminal practices, to reduce peoples to servitude, to torture persons or put them to death. The misuse of God's name to commit a crime can provoke others to repudiate religion.
Blasphemy is contrary to the respect due God and his holy name. It is in itself a grave sin.79

2149 Oaths which misuse God's name, though without the intention of blasphemy, show lack of respect for the Lord. The second commandment also forbids magical use of the divine name.

[God's] name is great when spoken with respect for the greatness of his majesty. God's name is holy when said with veneration and fear of offending him.80

IN BRIEF

2160 "O Lord, our Lord, how majestic is your name in all the earth" (⇒ Ps 8:1)!

2161 The second commandment enjoins respect for the Lord's name. The name of the Lord is holy.

2162 The second commandment forbids every improper use of God's name. Blasphemy is the use of the name of God, of Jesus Christ, of the Virgin Mary, and of the saints in an offensive way.

In the “Summa Theologica” (Secunda Secundæ Partis, 13 , 2), St. Thomas Aquinas discusses the sin of blasphemy.

Article 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. (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.

Footnotes

72 ⇒ Ex 20:7; ⇒ Deut 5:11.
73 ⇒ Mt 5:33-34.
74 Cf. ⇒ Zech 2:13; ⇒ Ps 29:2; ⇒ 96:2; ⇒ 113:1-2.
75 John Henry Cardinal Newman, Parochial and Plain Sermons V, 2 (London: Longmans, Green and Co., 1907) 21-22.
76 Cf. ⇒ Mt 10:32; ⇒ 1 Tim 6:12.
77 Cf. 1 ⇒ Jn 1:10.
78 ⇒ Jas 2:7.
79 Cf. ⇒ CIC, can. 1369.
80 St. Augustine, De serm. Dom. in monte 2, 5, 19: PL 34, 1278.