Lamentations 3:29

He continues the same subject; for he describes to us men so subdued to obedience that they are ready to bear whatever God may lay on them. He then says that the sitting and the silence of which he spoke, so far prevailed, that the children of God, though in extreme evils, did not yet cease to persevere in their obedience. For it sometimes happens that those who have made some progress in the fear of God, give proof of their obedience and patience in some small trial; but when they are greatly tried, then breaks forth the impatience which they had previously checked. Then the Prophet teaches us here, that the children of God do not sufficiently prove their patience, when they bear with a calm mind a moderate correction, except they proceed to a higher degree of perseverance, so as to remain quiet and resigned even when the state of things appears hopeless.

By saying that the faithful put their mouth in the dust, he means that they lie down humbly before God and confess themselves to be as dead. The import of what is said is this: In time of extreme affliction the wise will put his mouth in the dust, while seeing things in such confusion that all his thoughts vanish away on account of the atrocity of evils; and thus he intimates that the wise would have nothing to say. To put the mouth, then, in the dust is to become mute, as though he had said, that the faithful shut their mouth, when they do not murmur against God nor abandon themselves to complaints, when they do not expostulate that injury is done them, nor allege what the unbelieving usually do when God deals severely with them. In short, to put the mouth in the dust, means to bring no complaints, and so to check ourselves that no clamorous words proceed from our mouth. Thus another phrase is used to set forth the silence mentioned before.