[And they shall be a scourge unto the people of this land]: This makes the future history particularly relevant to these Nephites. It is possible to understand the future persecutions at the hands of the Gentiles to be something that would happen to the House of Israel of the Old World. The Savior makes it clear that this applies to this land. These are events that will happen here, to the descendants of those who are listening.
[Nevertheless, when they shall have received the fulness of my gospel, then if they shall harden their hearts against me I will return their iniquities upon their own heads]: This is the promise of reversal. The Gentiles may have the upper hand for a while, but eventually God and the House of Israel will prevail. This is a reference to the mission of the Triumphant Messiah. It is important to remember at this point that while the Saints in the New World did particularly await the Atoning Messiah, they understood and also awaited the Triumphant Messiah. They would have understood the two to be the same, but with different missions at different times.