Dr. Luther’s Small Catechism lesson for the week of February 15, 2015:
[I] THE TEN COMMANDMENTS
IN THE PLAIN FORM IN WHICH THE HEAD OF THE FAMILY
SHALL TEACH THEM TO HIS HOUSEHOLD
“You shall not take the name of the Lord your God in vain.”
What does this mean?
Answer: We should fear and love God, and so we should not use his name to curse, swear, practice magic, lie, or deceive, but in every time of need call upon him, pray to him, praise him, and give him thanks. 
 Tappert, T. G. (Ed.). (1959). The Book of Concord The Confessions of the Evangelical Lutheran Church. (p. 342). Philadelphia: Mühlenberg Press.