"Thou shalt not take the name of the Lord thy God in vain" (KJV; also "You shall not make wrongful use of the name of the Lord your God" (NRSV) and variants) is one of the Ten Commandments.
It is a prohibition of blasphemy, specifically, the misuse or "taking in vain" of the name of the God of Israel. Exodus 20:7 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." (KJV).
Based on this commandment, Second Temple Judaism by the Hellenistic period developed a taboo of pronouncing the name of God at all, resulting in the replacement of the Tetragrammaton by "Adonai" (literally "my lords" – see Adonai) in pronunciation.
In the Hebrew Bible itself, the commandment is directed against abuse of the name of God, not against any use; there are numerous examples in the Hebrew Bible and a few in the New Testament where God's name is called upon in oaths to tell the truth or to support the truth of the statement being sworn to, and the books of Daniel and Revelation include instances where an angel sent by God invokes the name of God to support the truth of apocalyptic revelations. God himself is presented as swearing by his own name ("As surely as I live …") to guarantee the certainty of various events foretold through the prophets.
–Wikipedia