what does " taken in vain"here mean?
"That's my picture," said a little voice in the room. Nat popped up his head, and there was Demi in his night-gown pausing on his way back from Aunt Jo's chamber, whither he had gone to get a cot for a cut finger.
"What is he doing to the children?" asked Nat.
"That is Christ, the Good Man, and He is blessing the children. Don't you know about Him?" said Demi, wondering.
"Not much, but I'd like to, He looks so kind," answered Nat, whose chief knowledge of the Good Man consisted in hearing His name taken in vain.