Good philosophers, like good scientists, should share their raw data, warts and all. (Yes, that's a dig at Phil Jones.) In the interests of full disclosure, what would change about my life, were I suddenly to become a materialist?
Pursuing my own pleasures would suddenly take a higher priority. If physical death is the end of life, then one's calculations for deferral of gratification could change significantly. No more "laying up treasure in heaven," doing good deeds with the thought that one will be rewarded in the afterlife. I wouldn't even wait for retirement, much less death; I'd make darn sure that I had rewarding experiences along the way, since tomorrow might be the end. As Kenny put it, more chess and movies, and less meditation. But again, I don't think that necessarily changes my lifestyle much – I deliberately developed a life in which my work and my play were pretty close to the same thing.
I think what we might term ordinary morality -- a basic respect for other human beings, generosity towards your immediate friends and neighbors, an aversion to crime -- is totally intact. No one needs a God in order to naturally do these things. I would still be trustworthy, give blood every couple of months, and help out a stray dog now and then.
What takes a real hit is the superrogatory goodness -- goodness above and beyond the call of duty, goodness that involves sacrificing my own self-interest. I would not "give 'till it hurts," as the Red Cross asks. I would be a decent person, but most likely I would not be a saint. It would still be possible for me to give generously to causes in which I or my family had a vested self-interest (like volunteer work at the school) or in which I had a personal intellectual interest (like technology work at the school or the SKS) or which provided other compensations like interesting friends (school and SKS again). But I wouldn't consistently put those causes ahead of my family's interests, or if they got to be unpleasant or felt unrewarding I would probably drop them.
Or, to put it another way: I would stop doing good things that I thought I ought to do, and only do those good things that I felt like doing.
The loss of the prospect of sainthood is significant. I'm currently reading David McCollough's history 1776, and I am acutely aware of how much I owe to people who made extraordinary sacrifices to establish the society I live in. I'm not sure if our way of life could have come to be, or be maintained, without people who live and die for something outside their narrow personal interests.
Oddly, and ironically, all good deeds (those I do for others, and those others do for me) would be imbued with a greater significance than if I was counting on a spiritual reality. If a Christian does you an unexpected good turn, it's nice, but you know they are just investing in their immortality 401k plan -- it's just a rarified form of self-interest. But when an atheist does you an unexpected good turn, it means a heck of a lot more. That person just gave you time and energy they will never get back again, for no other reason than they really wanted to. What a gift. What a pure act of self-expression. I am reminded of Angel, Joss Whedon's vampire-with-a-soul who discovers that no amount of good deeds can win him ultimate redemption, and who still does good anyway, because, in his words, "What else are we gonna do?" When he faces despair and has an epiphany that leads him back to life, he sums it up thus: "If nothing you do matters, then all that matters is what you do -- right now." Somehow it makes more sense to me when he says it, instead of Nietzsche or Sartre.
Imagine that you see a rock, teetering on the edge of a cliff. There is no one below the cliff, who might be hit. There is, in fact, no one around but you. It's not a particularly big rock, not part of any structure, etc. So you can go put the rock back on the cliff in a more stable place, or you can throw it off the side of the cliff, or you can just leave the whole thing alone. Here is my question: do you have any moral or ethical reason to do any of the above?
Well, sure you do, if you imagine that the rock is trying to launch itself off the cliff. That's what it looks like, when you look at it that way: like Tantalus, it keeps straining for the edge, but can't quite get there. Life is probably a lot better down at the bottom of the cliff, it reasons. So you really ought to help it out and give it a toss.
But of course, all that is completely fanciful. Really, the rock doesn't care one way or the other. You get no ethical high marks, or demerits either, no matter what you do.
And here's the point of this unnecessarilly-lengthy analogy. In a purely material world, everything and everyone is just like that rock. A small child is "trying" to get some food? Not really, that's just a fanciful way of putting it. What's really happening is a complex series of mechanical, electrical, and chemical reactions that all lead the child to move its legs, hold out its arms, and act for all the world as if it really matters if it gets the food. Those physical events are the whole story: anything else we add to them is self-indulgent anthropomorphism. Maybe the child gets the food and lives, maybe it doesn't and dies, and as Dr. Manhatten says, a dead body contains roughly the same number of molecules as a living one, so what's the diff?