The second commandment is “You shall not take the name of the Lord your God in vain.” It commands us to respect the name of God, be truthful in oaths, and be faithful to vows. It forbids using the name of God in vain, blasphemy, cursing, perjury, and violation of a vow.

Using the name of God in vain means pronouncing it without good reason and without due respect; for example, using it to express surprise or anger. Ordinarily, using God’s name in vain is a venial sin.

Blasphemy is an expression insulting to God, to religion, or to the saints. Blasphemy is a mortal sin, and it can be venial only through some defect of inadvertence or of will.

Cursing is the calling down of some evil on a person, place, or thing. To wish a small evil on a person is a venial sin, whereas to wish a great evil on a person is a mortal sin. The worst curse is to wish a person be condemned to hell.

However it is permissible to wish, without cursing, some physical evil on another, provided the intention is the person’s spiritual welfare.

For example, after the death of her husband, St. Rita discovered that her two sons were thinking of avenging their father’s murder. She begged God to take them rather than permit them to commit this mortal sin. Not long afterward they both died, after preparing themselves to face God.

Bishop LaRavoire Morrow, author of My Catholic Faith, wrote, “When angry, parents sometimes curse their children, and workmen their animals and tools. Often the one who curses does not mean what he says. If he does, it is indeed a most grievous sin to ask God to damn a person or send him to hell.”

Truthful in oaths

An oath is the calling on God to witness the truth of what we say. Taking an oath is called swearing.

According to the Baltimore Catechism, to make an oath lawful, three things are necessary: first, we must have a good reason for taking an oath; second, we must be convinced that what we say under oath is true; third, we must not swear to do what is wrong.

Deliberately taking a false oath is the sin of perjury. It is always a serious sin because it implies a desire to destroy God’s truthfulness. Taking a true oath without necessity is usually a venial sin.

Faithful to vows

A vow is a voluntary promise made to God to do something that is better than what we are obliged to do under pain of sin.

Father John Hardon, S.J., author of The Question and Answer Catholic Catechism, wrote, “A vow is pleasing to God because it unites the person to God by a new bond of religion. Moreover, it means offering up to God not only a single act, but the will behind all the acts performed under the vow.”

The most important vows are religious vows, taken by those joining a religious order: vows of poverty, chastity, and obedience.

Before taking a vow, we should weigh the matter carefully, and not act rashly. It would be good to ask the advice of a priest.

If a person is unable to fulfill a private vow, he should consult his confessor about having it annulled or changed into some other good work.

“If we consider that God has given us a tongue with which to pray and sing His praises: that we have so often used it as a halting-place for the Saviour Himself, how could we use it to outrage so good a God, and to profane so holy and so honourable a name?” (St. John Vianney).

「天主十誡」中的第二誡是「毋呼天主聖名以發虛誓」，命令我們要尊崇天主的聖名、誠實發誓、還所許的願，禁止我們妄用天主之名、褻瀆天主、詛咒、發假誓、許願卻不還願。

沒有充份及嚴重理由而輕率利用天主之名，又或言不由衷，缺乏尊重 —— 例如用以表達驚異或忿怒，均屬觸犯此誡。不過，基本上是小罪。

褻瀆是指侮辱天主、侮辱宗教或聖賢；明知故犯的褻瀆是大罪。

詛咒是意願人物、地方或物件，遭逢邪災之害：意願小災是小罪，意願大災是大罪。意欲他人下地獄是最大和惡毒的詛咒。

不過，假如是為了別人的靈魂的益處，祈求天主的降下肉身上災禍，沒有詛咒之心、惡毒之意，則是允許的。聖婦麗達丈夫被謀殺，她兩名兒子矢志復仇，聖婦向天主祈求，但願兒子死去都不願見到他們犯罪；果然不多久，她兩名兒子得蒙善終之恩。

「我的天主教信仰」作者蒙諾主教寫道：「在忿怒中，父母可能咒罵兒女、工人咒罵奴畜或工具，一般是言非由衷；但如果是蓄意的詛咒人下地獄，則是嚴重的大罪。」

誠實發誓

誓是向天主發的—–要求天主作真實言詞的見證。據博迪模天主教要理指出，使發誓合法要有三個條件：一、有充份和重要的理由，二、內容真實不偏，三、不要為作惡事而發誓。

發虛誓是大罪，因為那是詆譭天主的忠實毋虛。另方面，在不必要情況下發真實的誓是小罪。

忠於所許的願

許願是自發性地向天主承諾，去做更中悅祂的事，也定下不做有罪。

耶穌會會士若望.赫頓神父在他的「天主教要理問答」一書內寫道：「許願是鍾悅天主的，因為它使人與天主藉宗教之德而結合。再者，我們獻上的不僅是一個善行，但也獻上在聖願下推動善行的意志。」

許願中最重要的莫過於修會的聖願。修道的修士和修女發的聖願是：神貧、貞潔和聽命。

許願不能草率，我們要先衡量自己的能力和意志。還是最好先請教神長。

一旦不能實踐私下發的願，應與請聽告解神父商討，好能解除所許的願，或由他指導更改所許的願。

聖若翰.維雅神父說：「天主既然賞給我們口舌去祈禱和詠頌祂：而我們又時常用它作救主的宿地；為何我們要用它來觸怒萬善的天主，並用來褻瀆祂至聖至尊之名？」