In a recent interview (Inquisitive Minds postcast, part 2), atheist Hector Avalos says:
It's no virtue in doing good because you're tying to please an invisible being. There's no reward that's going to come to you. That's not really moral action.
That sounds oh-so idealistic. But let's be honest. What reason does an atheist have to be sacrificial? From a secular standpoint, isn't that irrational? Where's the evidence that Avalos practices moral heroism?
It also depends on what you mean by "reward." There's the quid pro quo sense of reward, where you do something in exchange for what you receive in return. You scratch my back and I'll scratch yours.
But that's not what's meant by eschatological rewards. To begin with, to be a creature is to be dependent. To have physical and emotional needs. Humans aren't robots. There's nothing intrinsically unspiritual about enlightened self-interest when it comes to having your essential needs met. That's actually a pious, humble acknowledgement that you're not God.
"Eternal life" is eternal happiness. Although duty takes precedence over happiness, you can't be depressed all the time and survive emotionally. Without hope, existence becomes unbearable. Hell is everlasting despair.
In addition, the question of whether virtue is "rewarded" and vice is punished goes to the issue of whether there's any ultimate justice. If doing good is an infallible recipe for getting screwed, it is foolish to keep doing good.
It sounds nice to say we should simply do good for its own sake, come what may, but if the universe was rigged so that do-gooders always lose and always suffer while evil-doers always win and always prosper; if–without fail–evil is rewarded and virtue is punished, it would be irrational to do the right thing. We need something to look forward to. We need to get something out of life. If not this life, then the afterlife. Interminable misery is nothing to live for. That's damnation.