Jesus is referring here to the fact that Lehi’s family fled Jerusalem because of the city’s sin, which resulted in the destruction of the temple and the exile of many people to Babylon. Jesus teaches the Nephites that their ancestor Lehi was separated from the rest of the House of Israel because of “their” iniquity—a pronoun that is repeated in this verse and again in the next. In other words, these people are descended from a righteous remnant and have a noble heritage (whatever they have done in the subsequent generations).