Chapter 35: Lord's Day 35
Section 99. Q. 99: What is required in the third commandment? A. That we must not profane or abuse the name of God by cursing, perjury, or unnecessary oaths, nor share in such horrible sins by being silent bystanders. In a word, we must use the holy name of God only with reverence and awe, so that we may properly confess him, pray to him, and praise him in everything we do and say.
Section 100. Q. 100: Is blasphemy of God's name by swearing and cursing really such serious sin that God is angry also with those who do not do all they can to help prevent and forbid it? A. Yes, indeed. No sin is greater, no sin makes God more angry than blaspheming his name. That is why he commanded the death penalty for it.