But, if you have no faith in the value of humanity as a source of goodness, then why should humanity be preserved? What is the point of continuing our existence if it mostly does harm?
This is such an interesting statement. As MC Hammer would say, let's "break it down".
Humanity as a source of goodness. Well, it is a source of goodness. It is also a source of evil. Not simply are we simplistically oppositional is our society, we have many variances such as strength, courage, weakness and fear. But humanity, in my opinion as a whole, is a source of self preservation. We react to stimulus that act on us. We learn from our experiences, and try to improve our lot in life. But humanity, without deity, is simply people trying to get to the next day. All sorts of people in a great big earthen stewpot, swimming around, keeping their head above water.
Why should humanity be preserved? Because it does, and will do so until something greater than humanity changes things. Of course, humanity has the option of unpreserving themselves when science and technological growth exceeds the growth of the moral compass and ability to self moderate.
I think that is our greatest gift and our worst detriment, self preservation. We want what is best for our selves and our family and friends and our state and our country and our hemisphere,and our world and our solar system and our universe and so on. All the later can be pitched when what's best for our own personal lives gets called into question. Not that hard to forget about universal well being if our own life must change, negatively, or increase our burden of work to make it so. The same argument holds true until you get on down the list. That is where things get interesting.
Some people will throw out everything down to their own state, while others may stop with only themselves. Without the guidelines Christians find in the Bible, selfishness is the only logical answer for humanity. How selfish depends directly on the individual's perception of his area of responsibility and how safe their environment.
Feeding others that can offer you nothing in return doesn't compute. It might be nice to help out a widow if it doesn't put me out too much money, time, or really anything that is too important for me to part with. I call this the warm and fuzzy syndrome.
And if times get really tough, as in great depression tough, then people, without moral conscience, have to figure out how far up that scale to push to get what they need. This leads to intimidation tactics, hostility, and other rotten stuff. You only do what you have to to survive. It's easy to rationalize it if you have to do it to survive, or maybe to feed my family, or maybe take care of my own self and business. It's a sliding scale that is deferent for everyone.
These are things going on in humanity in a micro and macro sense, constantly.
Historically, we see micro examples of humanity rising and falling. Greece, Rome, France, Great Brittain, USA, all are examples of greatness begetting greatness, humanity building upon itself, and varying degrees of that greatness not being enough to sustain itself indefinitely.
But when you break it all down, individually, systematically, I think you will find, that the answer to your question is that when we stop being good enough, long enough, humanity will not sustain. Whether the goodness of humanity comes from God through Jesus, or some faith of mankind; when things are no longer good enough, they fail.
Ask Stanley Kirk Burrell about his music career and you'll probably agree.
RCF