If one believes that nothing matters, that existence is objectively meaningless, that there's no inherent purpose to anything, the logical implications of that are a life of nihilism and hedonism.
Yet I am an atheist who is neither a nihilist nor a hedonist. Reason is a tool for managing the irrational experiences that give life meaning, and it does not lead me to your conclusions.
Incidentally, the most nihilistic people I encounter are very religious, and come from religions that preach that man is a failed creature utterly dependent on the mercy of a god and worthy of extinction at birth due to an inborn and inherent disease of the soul absent grace. They also learn that the world is a horrible place, and not to mingle with it or allow it to influence you. They are taught that the universe is base matter rightly slated for annihilation, that flesh is vile and and corrupts the spirit, and that even their own minds can't be trusted, that cognitive dissonance and doubt are the work of an evil spirit infecting the mind, and that life should be lived as if one is waiting at some cosmic bus stop to die and go somewhere better.
What's more nihilistic than that?
And then when I disagree, saying that I enjoy being an active member of society (at least I did until recently), that I enjooy the company of many of them, and that my life has been rich and satisfying because of that involvement, I am accused of being selfish what with there being so many less fortunate people in the world, as if I ought to be unhappy, too, for that reason.
This is essentially the opposite of the transition you described going fro atheism to theism. You described a positive impact on your worldview.
But look at how much faith has damaged this person, not atheism, so the idea that a god belief will enrich life's experience, which wasn't true for me (life got better after the transition from Christianity to secular humanism), certainly isn't true for the religious nihilist. In this case, it degraded a life (I'm thinking of a specific RF poster now).
If all we have is this life, and in the final analysis nothing any of us do is going to matter, then you have no legitimate reason to do anything but serve your own self-interest.
Matter to whom or what? The universe? I don't care what matters to the universe. What I do matters to me.
And yes, I serve my own interests. That is what we do by instinct.
My self-interest is in having a smooth, comfortable, stimulating, and meaningful life rife with satisfaction and void of shame, blame, guilt, loneliness and the like. I have discovered that the path to such a state involves loving others, earning their respect and affection, being of service, living an upright life free of lying ad deception, etc.. I do all of these things for me because they feel good and lead to a better life, but others benefit as well, and that is part of my satisfaction, too.
And I believe other approaches to life such as hedonism in the most derogatory sense, will diminish my life experience. Win-win.
if the ultimate goal of morality, as a product of evolution, is to help me pass on my genes, then sacrificing my life is completely contrary to that goal
Once again, I don't care what the universe's "goals" might be. I care about what my goals are. If I want to risk my life for another because I am compelled to do so, then that is my goal, not passing on my genes, which was never a goal of mine ever. I wanted children to love them and for them to love me, to increase my sense of purpose and meaning, and to make a lasting and constructive contribution in their lives. I don't care if they share my DNA. That's an accident of biology.
Good thread.