What does grandfather want Isabel to find?

In one of the most notable passages of the book, Grandfather urges Isabel to choose her own side in the war. Wisely, he leaves it up to her to decide which side she should support in this increasingly bitter, drawn-out conflict. Only that way, he says, will Isabel be able to find her way through the valley of darkness to the River Jordan.
Grandfather uses biblical imagery here to emphasize the importance of the decision that Isabel must make. In the Old Testament, the River Jordan was the place which the Israelites must cross if they're to enter the Promised Land. The implication here is that, whichever side of the conflict she chooses to endorse, Isabel must not lose sight of what's most important: achieving freedom from slavery.