(Vers. 29.). For while they trust in idols, which are without soul, as regards life: swearing wickedly, namely on the part of the thing by which they swear, because by an idol, and on the part of the thing for which they swear, because it is false, against that saying of Jeremiah 4: "You shall swear: The Lord lives, in truth and in judgment and in justice." They do not expect to be harmed, that is, they do not fear.
But how do they not fear, since they believe them to be true gods?
It must be said that although they worship them as gods, they nevertheless do not think omnipotence to be in them; or they do not believe that they punish the sins of men. And in truth they cannot harm them, on account of which Jeremiah 10: "Do not fear them," namely the idols, "for they can do neither evil nor good"; but nevertheless they will be punished by God for the sin of perjury.
(Vers. 30.). Whence follows: Both things therefore will come upon them, namely the evil of guilt, because they are permitted to swear wickedly, and the evil of punishment, because they will be punished for this: they will come upon them, I say, deservedly, namely according to their merits: Apocalypse, last chapter: "He who is filthy, let him be filthy still." Because they thought wrongly of God by attending to idols, that is, by attributing divine worship to idols, while they swear by an idol. And they swore unjustly by an idol, despising justice, namely by swearing what is unlawful or what is false, and thus they err or sin in a twofold way, namely, because they swear by what they ought not, and because they swear that which they ought not: Exodus 23: "You shall not swear by the name of foreign gods."
(Vers. 31.). For not, as if to say: and truly they sin by swearing thus: for not of those who swear, namely in this way, is the virtue, that is, a virtuous or meritorious work; but the punishment of those who sin, that is, the sin itself, which is also the punishment of preceding sins and guilt in itself. It always walks about, namely from bad to worse, in the transgression of the just, that is, of just things, namely of God's commandments; Gregory: "For a sin which is not washed away through penance, by its own weight soon drags one to another": for in the Psalm: "Let their way be darkness and slippery" etc.