The Third Commandment also has not fared well in English. Lo tissa et shem Ha-Shem Eloheikha la-shav is usually translated as "You shall not take the Lord your God's name in vain." Many people think that this means that you have to write God as G-D, or that it is blasphemous to say words such as "goddamn." Even if these assumptions are correct, it's still hard to figure out what makes this offense so heinous that it's included in the document that forbids murdering, stealing, idolatry, and adultery. However, the Hebrew, Lo tissa, literally means "You shall not carry [God's name in vain]"; in other words, don't use God as your justification in selfish causes. The Third Commandment is the only one concerning which God says, ""for the Lord God will not forgive him who carries His name in vain" (Exodus 20:6-7). The reason now seems to be clear. When a person commits an evil act, he discredits himself. But when a religious person commits an evil act in the name of God, he or she discredit God as well. And since God relies on religious people to bring knowledge of Him into the world, He pronounces this sin unpardonable.
Rabbi Joseph Telushkin, Jewish Literacy









