See also: Exodus 20 (Ten Commandments)
As regards to profanity or taking the name of the Lord in vain, the names of Deity should be used only in prayer or in dignified address or speech, and certainly never in needless or careless utterance. To use the usual swear words is bad enough – they brand one as crude and careless – but to use profanely any of the names of our Lord is absolutely inexcusable. Should one ever slip in this way he should repent in “sackcloth and ashes,” the same as if he had committed any one of the other serious sins. — President Spencer W. Kimball, Miracle of Forgiveness, p. 54
Beyond the counsel of the prophets and apostles – both modern and ancient – to avoid profanity our speech should be pure and refined. Other forms of speech are just as hurtful: gossip, backbiting, disloyalty, the “put down,” constant complaining about things or events we have no control over. This language is just as vile as the swear words dotting our daily reading and listening fare. — The Church News, February 8, 1992, p. 16
Profanity takes its toll on the one who uses it. As we read in Proverbs, “A wholesome tongue is a tree of life: but perverseness therein is a breach in the spirit” (Proverbs 15:4). The Spirit of the Lord, the Holy Ghost, testifies of God the Father and his Son, Jesus Christ. (See 2 Nephi 31:18.) When those names are dishonored, that Spirit, which “doth not dwell in unholy temples” (Helaman 4:24), is offended and withdraws. For this reason, those who profane the name of God inevitably relinquish the companionship of his Spirit. — Elder Dallin H. Oaks, Ensign, May 1986, p. 51
It is difficult to understand how a person may truly and sincerely approach God in prayer, seeking a blessing at his hand, at the same time be so disrespectful as to take his name in vain.
Profanity is incompatible with reverence. Surely at this critical time in our nation’s history, when we need the sustaining help of God, we should see that we do not offend him by reason of our language. We appeal to our young people everywhere to hold in reverence the sacred name of Deity, that they may walk acceptably before the Lord, so that, should there come a time in their lives when they need his sustaining help, they may go to him with good conscience and call upon him with faith that he will hear their plea. (LeGrand Richards, “The Third Commandment,” The Ten Commandments Today, pp. 52–53) — Old Testament Student Manual, p. 129