The full transcript of this 1991 debate is here on the Reasonable Faith site. Keep in mind that this is young William Lane Craig against old, experienced Kai Nielsen.
Nevertheless, here is an exchange I wanted to highlight.
William Lane Craig:
The chief purpose of life is not happiness per se, but the knowledge of God. One reason the problem of evil seems so puzzling is that we tend to think that the goal of human life is happiness in this world. But on the Christian view this is false. Man’s end is not happiness as such, but the knowledge of God–which in the end will bring true and everlasting human fulfillment. Many evils occur in life which seem utterly pointless with respect to producing human happiness, but they may not be unjustified with respect to producing the knowledge of God. Innocent human suffering provides an occasion for deeper dependency and trust in God, either on the part of the sufferer or perhaps those around him. Whether God’s purpose is achieved through our suffering all depends on how we freely respond.
Kai Nielsen:
The first thing you should come to recognize is that there can be purposes in life that are perfectly intact even if there is no purpose to life. If there is no God or telos of any sort, there is no purpose to life; you weren’t made for a purpose. But even if you weren’t made for a purpose, you could find plenty of purposes in life, things worth doing and having and believing and struggling for. Some religious people will say, “That’s all right for little individual small purposes, but you can’t have any overarching purpose in life without belief in God. You can little, fairly trivial things, but no really deep and pervading conception of a purpose in life without God.” But that’s not true. There are many atheists who have had such overarching purposes. They’ve fought relentlessly “the plague” (to use Camus’ metaphor). They’ve sought to lessen the sum total of human suffering, of human degradation, of blighted hopes; they’ve positively sought to bring about a world with more happiness in it and more understanding of each other–more human flourishing, more human solidarity, a brotherhood and sisterhood. They’ve sought, in short, to bring about a classless, raceless, genderless world. Those big purposes – overarching purposes in life – are perfectly available to anyone who is an atheist as well as to someone who is a theist. I don’t deny that believers haven’t done that, but you don’t need God to have either small purposes in life or an overarching purpose in life.
William Lane Craig:
First of all, I have never denied that you can have subjective purposes in life, but what I am arguing is that there is no objective basis to assume the moral worth of your purpose in life on an atheistic view. All purposes in life you choose are morally equal–whether you want to live a life as a doctor caring for the poor or choose instead to be a Ferdinand Marcos. There’s no objective basis for assessing the moral worth of those purposes.
Kai Nielsen:
He says that I don’t meet his objection that moral purposes in life all are perfectly arbitrary without God. I don’t see the slightest reason for that. Some of them are arbitrary if they are silly and thoughtless purposes. Some purposes, if they are integrated, carefully thought-out, related to everything else we know, reflective (where human beings work through long traditions, including working with Christian traditions, in which you put together everything you know and think carefully about these purposes), are perfectly objective. And they are the only kind you can give much sense to.
You’ll note that what counts as a good purpose on Kai’s view is not objective. Every purpose that he mentioned (e.g. – genderless world) was meaningful to him. He chose them because they made him feel good. But do these purposes have any significance, ultimately?
Consider the heat death of the universe:
The ‘heat-death’ of the universe is when the universe has reached a state of maximum entropy. This happens when all available energy (such as from a hot source) has moved to places of less energy (such as a colder source). Once this has happened, no more work can be extracted from the universe. Since heat ceases to flow, no more work can be acquired from heat transfer. This same kind of equilibrium state will also happen with all other forms of energy (mechanical, electrical, etc.). Since no more work can be extracted from the universe at that point, it is effectively dead, especially for the purposes of humankind.
This is how the universe ends, on atheism. No purpose achieved by any atheist matters, ultimately – the end destination is the same. Humans are going to die out individually and collectively on atheism, nothing anyone does to affect anyone else is going to change a thing. Nothing atheists do matters.
Atheists like to make much of this life being all they have, and to live it to the fullest, etc. Their purposes are carefully chosen to give them happiness in this life. Every purpose is as valid as any other, so long as pursuing that purpose produces happiness for the pursuer. But nothing ultimately matters if atheism is true, because the end result of the universe is the same. Nothing they do matters to them, because they’re dead. Nothing they do matters to anyone else, because we’re all dead. It will be as if we never existed or did anything, one way or the other. The only reason to adopt a purpose, on atheism, is for the happiness it brings you in this life. The purpose cannot be chosen in order to have ultimate significance, because on atheism there is no ultimate significance – everything ends in the heat death of the universe regardless of what we do.
In Christian theism, however, what you do in this life has ultimate meaning beyond our earthly lives. What you choose affects your afterlife. What you do with others affects their afterlife. Plans made with respect to that objective goal of knowing God do stand the test of time. They matter, even if they don’t feel good now. If your actions help to turn someone towards God or away from God, it makes a huge difference. A difference that lasts.
WINTERY KNIGHT 
The one question I don’t think any fatalist atheist will ever answer the question: “Why not kill myself?” Beyond some shallow, “just because” I don’t think on a long timeline there is any reason to survive even another minute.
If there is no afterlife, then there is no difference between a baby dying minutes after being delivered or amassing tons of knowledge and wisdom at the longest lifespan. All the experiences and knowledge and even a persons legacy will eventually be forgotten and practically for nothing.
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I think any answer that an atheist gives to the question of purpose is going to have to involve base animal desires for pleasure. Since nothing they choose to do for “purpose” has any ultimate meaning, because of the heat death of the universe, they have to choose something that gives them pleasure now, like any animal would. Since there are clearly cases where the desire for pleasure conflicts with the the obligation to love others self-sacrificially, they can’t really engage in anything noble or worthwhile, can they? Acts of self-sacrifice are completely irrational on atheism, and relationships that require self-sacrifice, like marriage and parenting, suffer as a result. When something isn’t rational, then people don’t do it, especially when it’s hard to do.
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Even living to amass pleasure doesn’t sway fix things. All the pleasure is forgotten when you cease to exist. All the comfort is ruined while dying. I know you’re not the biggest G.K. Chesterton fan, but I think when it comes to pleasure he has a great quote. “Despair does not lie in being weary of suffering, but in being weary of joy.” If that pleasure becomes dull or boring, one is left meaningless.
“Since there are clearly cases where the desire for pleasure conflicts with the the obligation to love others self-sacrificially, they can’t really engage in anything noble or worthwhile, can they?”
Funny, I just looked up a quote from the Marquis de Sade (the name you get Sadism from)
“Nature has endowed each of us with a capacity for kindly feelings: let us not squander them on others. What to me are the woes that beset others? Have I not enough of my own without afflicting myself with those that are foreign to me? May our sensibility’s hearth warm naught but our pleasures! Let us feel when it is to their advantage; and when it is not, let us be absolutely unbending. From this exact economy of feeling, from this judicious use of sensibility, there results a kind of cruelty which is sometimes not without its delights. One cannot always do evil; deprived of the pleasure it affords, we can at least find the sensation’s equivalent in the minor but piquant wickedness of never doing good.”
– Philosophy in the Bedroom
I find this useful for the Atheist who claims objective moral duties in advancing human well being. The first question should be who’s well being? and the second should be why waste my well being on others?
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Yes, these are the real questions. When I talk to atheists who want to claim morality, I find that it’s just a very transparent attempt to be liked and to fit in. They don’t really want to think about morality very practically, and ask how this works for people who have that different sense of well-being or flourishing. Let’s face it, people have different ideas about how society should run in order to maximize well-being and to flourish.
Your point about why engage in self-sacrificial behavior to promote the well-being and flourishing of a society is decisive. Why would any atheist who is going to die anyway sacrifice his pleasure right now for a society that’s going to die a relatively short-time after him? The thing is, atheists just redefine morality to mean doing whatever makes them happy – that’s why they never think about self-sacrifice in moral questions. The answer to moral questions is self-evident: I’ll do what I feel like, if I can get away with it (getting caught would reduce their selfish pleasure). That’s what they mean by “morality”, and that’s why it never occurs to them how ridiculous their view is. They are talking about preferences and pleasure in a conversation about virtues and obligations. How were we designed to live? That’s what determines morality, and what makes going against our self-interest rational. If you don’t believe that we have any design, then morality is nonsense.
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I’m sorry if at this point I pester you (I love you, brother), but in what ways am I so philosophically different? Do I reek of laymanry? Do I need to be corrected at the foundation?
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Well, not knowing you I am sure that you are a good person and successful and hard-working. There’s really nothing for you to do, except to do an honest inquiry of the features of nature and history that point toward or away from theism, and just honestly fit them into your worldview. If you come out with atheism, great. But I really think that the list of factors that don’t sit well with naturalism is getting to be a pretty long and growing list, especially in the last 50 years. A good basic book on this is J.W. Wallace’s “God’s Crime Scene”. I was just re-listening to the intro and chapter 1 in the weight room today, to discuss it with a Christian single mom tomorrow. It’s a great book, and it will really appeal to anyone who just wants to believe what’s true. There is no heavy theology in it. IT EVEN HAS A GLOCK 21!!!! Don’t you like guns? I like guns. Not Glocks (phooey) but I like guns. He shoots criminals with guns, why wouldn’t you read his book?
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Sir, how did I give you the impression in what I wrote that I am not a Christian? I am a Christian and I’ve been reading Christian apologetics. I just needed help with holes in what I had written.
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Oh, sorry.
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Sir, I may be bothering you by pressing in, but what made you think I was an atheist?
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