One of St. Augustine’s enduring gifts to ethics has been Just War Theory. “Thou shalt not kill” comes with an asterisk and a long explanatory footnote. Augustine did not leave us a Just Lie Theory. “Thou shalt not bear false witness” is almost absolute.
Augustine wrote about lying because, of course, everyone does it. And not just about little things. Even Augustine’s co-religionists were saying anything they could to win converts to their side. This was bad. Lying about faith and salvation degraded and debased Truth, the foundation of Augustine’s spiritual values. Augustine worried that a person converted by a lie had never accepted the Truth, and so might not really be saved.