Gregory the Great, on Jer 9:5

“And a man shall mock his brother, and they will not speak the truth: for they have taught their tongue to speak lies: they have laboured to commit iniquity.”

“The insincere are to be admonished to learn how heavy is the labor of deception. They endure it with guilt. They are afraid of being found out. They are ever seeking dishonest defenses. They are agitated by fearful suspicions. But there is nothing safer to defend than sincerity, nothing easier to say than truth. For, when obliged to defend its deceit, the heart is wearied with hard labor. For it is written, "The labor of their own lips shall cover them." For what now fills them then covers them, since it then presses down with sharp retribution one whose soul it now elevates with a mild uneasiness. It is said through Jeremiah, "They have taught their tongue to speak lies, and weary themselves to commit iniquity." Said more plainly: "They who might have been friends of truth without labor, labor to sin."”

Imported from an open dataset — not yet checked against the printed edition.