Heidelberg Catechism 35.100

Chapter 35 — Lord's Day 35

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.