Exodus 20:7 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.
A common interpretation for theologians (though not as common for the average believer) is that the Third Commandment prohibits 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. Better translations translate Exodus 20:7 to mean: You shall not misuse the name of the LORD your God. NIV.
Jews treat the commandment as a prohibition against swearing falsely in the name of God by either:
- Saying something is true when it is known to be false.
- Saying something is false when it is known to be true.
- Saying something is true when it doesn't matter (trivial).
- Promising to do the impossible.
Typically the commandment is taken to mean one should not curse, especially curses with the word 'God' in them, although there is little textual basis for this interpretation.
The commandment could easily have served as a basis for making contracts, which due to the penalties and beliefs involved, would be more likely to be kept than a person's word alone. One could lend seed with payment of part of the harvest later and properly expect payment because they "swore to God". This utility would be perfect for early cultures, however, today it's mostly moot.