The Second Commandment is: Thou shalt not take the name of the Lord thy God in vain. It commands reverence in our speech towards God and all sacred things and it forbids irreverence in speech.  To use the name of God or of Jesus Christ as a prayerful salutation is a beautiful custom still preserved in some Christian countries. To use it irreverently and profanely as a common exclamation is a bad and sinful habit, and very regrettable in a follower of Jesus Christ. But it is usually a venial, not a mortal sin; as no insult to God is intended. St. Paul tells us such reverence is due to the holy name that: "In the name of Jesus every knee should bow." (Philip 2:10.) To use the Holy Name in an insulting way, or to speak with contempt and insult of God and sacred things is blasphemy; and blasphemy is always a mortal sin.

The other sins forbidden by the Second Commandment are: Cursing, and oaths that are false, rash, unjust or unnecessary. Cursing is calling down evil on oneself or one's neighbor; and it can be a mortal or a venial sin according to the nature of the curse uttered. A false oath is perjury, which is always a mortal sin; because by it we call the God of truth to witness to a lie. An unjust oath is always a mortal sin. because by it we call the God of holiness to witness that we are determined to sin by the practice of some injustice against our neighbor. We commit a sin by taking such an oath and another sin by keeping it. A rash oath is one taken without sufficient judgment of what it may imply; and hence it could be very serious and a mortal sin. Such is the Masonic oath, to keep secret any matters heard within the Lodge, with no previous idea of their nature. This oath could easily involve grave injustice to our neighbor.

An unnecessary oath is not as serious in its guilt as the others; it is generally a venial sin, and comes more under the nature of irreverence towards holy things. Our Lord tells us not to swear at all neither by Heaven because it is God's throne; nor by the earth for it is His footstool. But let your speech be yea, yea; no, no. (Matt. 4:34:37). That is, simply affirm or deny what you want to say without irreverence towards holy things. An oath is lawful and even an act of virtue, when it is taken for the sake of God's honor, our own or our neighbors good. Under this Commandment too, we consider the nature and it obligations of vows. A vow is a promise of a greater good made to God binding under sin. It may bind under pain of either mortal or venial sin.  We may make a  promise to God which we intend merely to bind like a good resolution. Such does not bind under pain of sin. Somewhat more solemn, but of the same nature, is a pledge to abstain from intoxicating drink.

Source:  Rev. W. Frean, C.SS.R. Commentary on the Catechism.