This happiness was not found in accomplishing something. It was found inside himself and in nature. Mill related to Wordsworth. I have a lifeguarding in-service to attend. I believe this is the realization that Mills experiences.
Happiness is more spiritual than reaching some goals.
They are just saying that it's an open question, and that it is wrong to try to compel belief upon them. The part about agnostics trying to ride the fence and be on the safe side is impossible. The problem with agnostics is they lack faith and need hard evidence. There is no way to prove this and lacking faith is no safety net. I feel as if I am rambling a little.