21. Second, the name of God ought not to be taken in vain in a promissory oath. Now a promissory oath is made in three ways, namely for promoting good, for preserving fidelity, and for entering into mutual peace. In the first way, inferiors swear to superiors; in the second way, equals to equals; in the third way, superiors to inferiors. By the first oath the Apostle swore, saying in Romans 1: God is my witness, whom I serve in my spirit. In the second way, Laban swore fidelity to Jacob. In the third way, superiors swear to protect the people. — From the first oath a threefold benefit results: faith in the intellect, love in the affections, and peace in effect. For these three reasons an oath is made for promoting good.
22. In the second way, a promissory oath is made for preserving fidelity, and this is done in three ways, namely for restraining malice, for settling controversy, and for clearing away infamy.
First, I say, it is made for restraining malice, so that a man may neither deny the true nor affirm the false; and this is the reason for the usefulness of the oath. — Second, it is made for settling controversy. Whence the Apostle says to the Hebrews: The end of all controversy is an oath.
Third, it is made for clearing away infamy. Whence in Deuteronomy it is said that the elders had to clear themselves by oath when a dead body was found.
For these three reasons an oath is made for preserving fidelity.
23. In the third way, a promissory oath is made for entering into mutual peace: and in this is understood the restraint of all mortal sins; because "every true oath is dangerous, a false one is pernicious, and no oath is safe." And for this reason it is said in Ecclesiasticus: Let not your mouth become accustomed to swearing. — This is the literal understanding of the words.
24. But the spiritual understanding of this precept is this: that we take the name of God not in vain in three ways, namely for effecting, for promising, and for expressing.
First, I say, we take up the name of God not in vain for effecting, as in the Sacraments, when it is said in baptism: I baptize you in the name of the Father and of the Son and of the Holy Spirit.
Second, we take up the name of our God not in vain for promising, as when I promise to go to Saint James, or to enter religious life, by an oath.
Third, we take up the name of God not in vain for expressing, as when we speak with an oath.
25. Conversely, we take up the name of God in vain in three ways: first, through frustration of a Sacrament received; second, through breaking of a vow made; third, through uttering of blasphemous speech.
First, we take up the name of God in vain through frustration of a Sacrament received; and according to this there is a threefold degree of taking the name of God in vain. The first is when someone does not believe in the Sacrament, like a heretic. Second, when one believes but does not dispose himself for the Sacrament, like a usurer. Third, when someone does not preserve the grace received in the Sacrament, like one who afterward sins. — Hear, you who are marked with the Christian name: the character is then imprinted upon you through the power of the name of God; and when you afterward sin, you empty out the power of the Sacrament. Of such persons the Apostle says to Titus: They profess to know God, but by their deeds they deny Him; and thus you trample upon the Son of God and treat as polluted the blood of the covenant, you who were washed in the blood of Christ!
26. The second mode of taking the name of God in vain is through breaking of a vow made. And here there is likewise a threefold degree. The first is through irrational delay, as when I promise to go to Saint James by an oath and do not assign myself a fixed term, and thus always procrastinate.
In the second way, breaking of a vow occurs through a contrary will, as when someone promises something to God and afterward repents of it. Against such persons it is said in Deuteronomy: If anything has proceeded from your lips, you shall not make it void.
In the third way, breaking of a vow occurs through apostatic rebellion, as when a man apostatizes from religious life.
The first of these modes is evil, the second worse, the third worst, because it is said: An apostate man is a worthless man; on account of which it is said in Ecclesiastes: If you have vowed anything to God, do not delay to pay it; for an unfaithful and foolish promise displeases God. And this is so when time and hour and day and year were calling out.
27. In the third way, we take up the name of God in vain through uttering of blasphemous speech; and this occurs in three ways: first, when someone, speaking of God, uses cursing speech, as when he curses God.
The second mode is through mendacious speech, when someone denies what is true of God, or affirms what is false; because Anselm says that the nobler part of every contradiction is to be attributed to God. - The third mode is through reproachful speech, such as when someone speaks of the lower members of Christ, he blasphemes most wickedly, even if he speaks the truth, and despises Christ in that He was humble and assumed our nature.
And such persons have been destroyed by divine judgment. For Gregory narrates that a certain boy of six years was accustomed to blaspheme the name of the Lord. One day, when he was blaspheming and was in the lap of his father, he was seized by demons and carried into hell. On account of which it is said in Leviticus: Whoever shall have blasphemed the name of the Lord, let him surely die.
It is now clear in what ways the name of God is taken in vain.