"My question is than what is there to beleive in just Nature? what purpose do we have?"
First off, "believe in just nature" is a loaded statement. If something is real, I need not believe it. Capitalizing the word "Nature" as you did shows that you put special meaning it that above simple reality. As I can not read your mind, my reply will ignore the emphasis you put on that word unless you want to clarify what you mean.
So, here's my answer;
The same purpose I've always had.
There is no mystery here and never has been.