The Third Commandment reads:
- 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)
As with other commandments, many believe that this third was originally quite a bit shorter than the version we have now. It has been argued that the earliest form would have simply been the first phrase, Thou shalt not take the name of the Lord thy God in vain, and the rest was added later perhaps by a priestly caste.
What does it mean to take the name of the Lord thy God in vain? That has been the subject of quite a bit of debate. Some have argued that this prohibition was limited to the use of the name of God in any frivolous manner. Others have argued that it was a prohibition against the use of the name of God in magical or occult practices.
A more common interpretation for theologians (though not as common for the average believer) used to be that the Third Commandment prohibited one from breaking any contract that was sworn in the name of God. Thus, if someone promises by the name of God to do something and then fails to do it, that would qualify as having taken the name of the God in vain.
There is strong textual support for this because the Exodus version can be translated as in a vain oath while the Deuteronomy version can be translated as in a false oath. Better translations of the Bible rarely say take the name of the Lord thy God in vain and instead say swear falsely by the name of God. That is not, however, what you will usually find on popular listings, for example those used in government displays or monuments.
Jews typically treat the prohibition against swearing falsely by the name of God to prohibit four different types of oaths: those that affirm as true something known to be false, those affirming as false something known to be true, those that affirm the truth of something obvious (due to its triviality), and those promising some act known to be impossible or outside ones abilities.
All of the interpretations above differ in many ways, but there is one common thread in that they forbid the use of the name of God in a manner that would dishonor God. It must be remembered that God is a jealous God and will allow no dishonor to come to him through the actions of his chosen people. To appropriate Gods name for deceitful or otherwise prohibited actions (as with magical spells) would amount to dishonoring God and Gods covenant with the Hebrews.
The most popular interpretation today is that the Third Commandments prohibits swearing, especially swearing that includes the word God, but there is little or no basis for such a reading of the text. You wont find many responsible theologians suggesting such an interpretation but you will find it in many popular texts.