In Noah's experience, we get what God's thoughts are of the things around. Noah was to be separated from the old earth. If we look around now, is there any city in which God's children are to find rest of heart? No! Only a place from which to separate themselves. Believers have to go through the world, but are to keep themselves unspotted from it.
By our very relations, we often find ourselves hindered and interrupted, and cannot get separate for want of faith. We need the energy of faith. Noah's energy all flowed from faith, following the line traced out by God; and when the judgment came, it found Noah in the ark, at rest there with his family. God saw in it the expression of his faith as a person separated by that faith to Himself.