Thou shalt not take the name of the Lord, thy God, in vain.
According to Luther's Small Catechism:
What does this mean?--Answer.
We should fear and love God that we may not curse, swear, use witchcraft, lie, or deceive by His name, but call upon it in every trouble, pray, praise, and give thanks.
According to Luther's Large Catechism:
If, then, it be asked: How do you understand the Second Commandment, or what is meant by taking in vain, or misusing God's name? answer briefly thus: It is misusing God's name when we call upon the Lord God, no matter in what way, for purposes of falsehood or wrong of any kind. Therefore this commandment enjoins this much, that God's name must not be appealed to falsely, or taken upon the lips, while the heart knows well enough, or should know, differently; as among those who take oaths in court, where one side lies against the other. For God's name cannot be misused worse than for the support of falsehood and deceit. Let this remain the exact German and simplest meaning of this commandment.