As it comes to faith, there are just certain things that are unknown: that's why it's called "faith." In the case of the Israelites crossing over the Jordan River, they were required to take a step of faith into the water, and it wasn't until the soles of the feet of the priests touched the water that they would be able to move forward. This act required great faith because if the water didn't cease, the priests and the Ark of the Covenant would have been swept down river.
Over and over again the Bible recognizes people of great faith who are willing to step out into the water, believing that what God has promised will come to fruition. Joshua wasn't being foolish in leading the Israelites to do this because he had been directed by God. It was still a risk, but it wasn't a foolish risk. There is a difference. Could God be asking you to take a risk somewhere with your faith? Does He want you to step into the water and get your feet wet? Just like with the priests, sometimes we will not see the promise fulfilled until we actually take that step.