The first two of the ten commandments instruct us to worship God exclusively and accurately. If we are obeying those two commands, then the third command should not be hard for us. “Thou shalt not take the name of the LORD thy God in vain; for the LORD will not hold him guiltless that taketh his name in vain.” (Exodus 20:7)
There are two ways that we can take the Lord’s name in vain. The first is by using God’s name as a by word. This is the most commonly thought of way that people violate this command. When someone says, “Oh my…” and then they put God’s name at the end of the expression, they are taking His name in vain. They are using God’s name like a cuss word.
The second way that you an take God’s name in vain is by taking God’s name to yourself (i.e. by calling yourself a “Christian”) and then not acting worthy of that name. 2 Timothy 2:19 sums up this idea thusly: “Let every one that nameth the name of Christ depart from iniquity.”
When someone takes God’s name in vain, they show that they have a flippant attitude toward God Himself. They don’t really take God seriously. The underlying principle of this third command can be summarized with the word “reverence.” We must worship God reverently.