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. 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. 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.

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.