"few adult persons can see nature" this is because only few people acutually see the beauty of nature the rest are in the main stream and dont pay attention to nature.

"...remembrance of the city of God which has been shown!" he is saying that there is a connection between nature and god

"trust thyself" this supports the idea of belive in yourself and then you can be self reliant.

"...it is easy in solitude to live after our own." this shows that it is not hard to step back from main stream America and look farther and look inside yourself.