The third thing God commands of us is not to take His name in vain. The word vain is defined on Dictionary.com as the following:

The idiom "in vain" is defined to mean "without effect or avail; to no purpose or in an improper or irreverent manner."[1]

To take the Lord's name in vain would be to use it without purpose, or in an improper and irreverent manner. Growing up the common examples of this were words/phrases that combined God with a swear word, and the phrase "Oh my God." The reason the first example is a breach of the commandment should be fairly obvious, it is irreverent. The case of the second may not be as obvious. This phrase is an expression of shock, akin to oh oh wow. I imagine, during the Exodus, when the Israelites watched the plagues befall Egypt, crying out "my God" would not have been a problem. They would be attributing the miracle to God. However if John Doe just asked you to the dance and you squeal "oh my God" to your friends, are you actually thinking about God? Probably not. This is a purposeless invocation of His name; it is careless and irreverent.

Mark 3 tells us that the only unforgivable sin is blasphemy against the Holy Ghost. in the dictionary, blasphemy is defined as cursing God, being irreverent toward God,[2] but John 10:33 also suggests that blasphemy is the act of making oneself God (this would still fall under irreverence, I suppose). To break the 3rd commandment is to commit blasphemy, as you are being irreverent towards God when you use his name in vain.

Mark tells us the reason for this is that the one who blasphemes has an unclean spirit. It is easy to see why God would be so angry at the misuse of His name. A simple walk through history will show where people twisting God's words and attaching God's name to these ideas has caused great suffering throughout the world as well as turned people away from God. This was the case in the dark ages, when the Catholic Church killed everyone who disagreed with them "in the name of God." This was the case when racist white people mangled the nature of the curse on Canaan, professing it to be a curse on Ham that they identified as dark skin, thus claiming God ordained Africans to be subservient, lesser people. Not only did both of these misuses of God's name continue for centuries and cause the deaths of many, it turned people (even today) away from God. Many people were bright enough to know that no just God would ordain and condone such thoughts and actions, however, instead of coming to the conclusion that these people were blaspheming the name of God, they stopped believing in God all together.