My response was to say I found what he said a bit hard to square with his atheism. You waste something when you don’t use it for its intended purpose, but if there is no God then life has no purpose – there’s no way you are meant to live. Everyone is free to invent their own purpose, but you can’t say that one is better than another, or that one person is investing his life wisely while another is wasting it. The atheistic worldview says your life has no ultimate significance.
The atheist said that the Christian was obviously wasting his life because he was living for something false, but in a godless universe there’s no duty to live for what’s true. Surely the atheist should be saying that you should spend your life doing whatever makes you happy, after all, once you’re dead, that’s it! Well, this Christian is spending his life doing something he believes is eternally valuable – that seems to be a far more emotionally satisfying way of spending one's life. As Puddleglum the Marshwiggle said to the witch, when he and his friends were trying to escape the Underland:
Suppose we have only dreamed, or made up, all those things – trees and grass and sun and moon and stars and Aslan himself. Suppose we have. Then all I can say is that, in that case, the made-up things seem a good deal more important than the real ones. Suppose this black pit of a kingdom of yours is the only world. Well, it strikes me as a pretty poor one. And that’s a funny thing, when you come to think of it. We’re just babies making up a game, if you’re right. But four babies playing a game can make a playworld which licks your real world hollow. That’s why I’m going to stand by the play-world. I’m on Aslan’s side even if there isn’t any Aslan to lead it. I’m going to live as like a Narnian as I can even if there isn’t any Narnia. So thanking you kindly for our supper, if these two gentlemen and the young lady are ready, we’re leaving your court at once and setting out in the dark to spend our lives looking for Overland. Not that our lives will be very long, I should think; but that’s a small loss if the world’s as dull a place as you say.
– C.S. Lewis, The Silver Chair
Even the atheist can’t escape the sense that life has purpose, but that is because he is created in God's image and for eternity (Genesis 1:26; Ecclesiastes 3:11). It turns out then that the only way to avoid wasting your life is to align it to the will of God (1 John 2:17).