What Mr. Richardson is doing here is referring to the sorts of ideas that will end up leading to the Revolutionary War. This conversation is taking place at a time when the colonies are still part of Great Britain.
When the colonists later started rebelling against Great Britain, they liked to say that the relationship between themselves and the British was one of slave to master. They liked to say that they needed to cast off the chains of slavery and become free. What Mr. Richardson is saying here is that he and the rest of the white colonists will themselves become slaves to the British if they are not careful.
So, what Richardson means is that white colonists will be enslaved by the British if they are not careful.