I'm debating a strong evangelical at work, and I've got him pinned to his strongest reason for belief: that there would be no purpose to his life without god. Of course I know that isn't true, but I want to respond to this thoughtfully. How would you respond to the statement there is no purpose without God?
Well, it an appeal to utility. His reason for belief is based on the utility or profit that this belief provides, in this case, he feels it provides him a purpose, he also think that only this particular belief can provide him a purpose. His belief is not based on evidence, reasoning, truth, etc... as many beliefs aren't, but it is based on wanting to be comforted, wanting to fulfill his desire to be motivated to live for something, to have an objective purpose. This raises red flags immediately when debating the verity of that in which he believes. Also, it is a selfishly motivated position for him to hold and not something the subject of his belief (God) would ever reward him for since it is based in selfishness.
And then of course there is the fact that he is mistaken to believe that his life would be purposeless without belief in a god. He would certainly have motivations and subjective purposes without belief god if not more. For one, start with the most basic instinct to survive. I would even dare to say he would indeed have no purpose WITH a belief in god, because he is a slave to this belief, to this god he believes in. Following only what he believes this god wants of him. What kind of purpose is that really? This belief in god steals his freedom and identity and makes him nothing more than a pawn moved on gods command. Does this motivate him and give him a sense of purpose and value, being determined by a god? It is not so different from a natural viewpoint on how we are determined by causality, by the conditions of our environment. Belief in a god doesn't offer purpose, on the contrary, it is a thief and steals ones real potential towards a pursuit of individual value and meaning in the universe.
Some would answer that our purpose is to be with God, but that's a self-fulfilling prophecy. So we create this idea, call it God, to justify the intangible parts of life and give us "purpose", but our purpose is merely just to BE with that very thing that we created just for the sole reason of having a purpose? Am I the only one that finds this a little creepy? Okay. Flip side. God creates us for the sole purpose of having a fan club and some company. But we're separated by something called real life and apparently invisibility. Why the purposeful separation? And what is our purpose while we're here? Isn't it the exact same reasons that people of other beliefs and no religions give themselves? HELLO!
I'm not sure that we can respond to the general statement, "there is no purpose without God" unless we define what that purpose might be.
First I'd ask him to identify what he considers his life purpose to be. (Carrying on the species, being kind to others, spreading the word of God, contributing in some meaningful way to the benefit of the planet...?)
Then you can tackle whether God needs to exist for his purpose to exist. If the purpose is something like being kind to others, then clearly no God is required (and you can explain how kindness and co-operation are moral qualities that are not dependent upon a higher power.)
If his purpose is to spread the word of God, you can still break down the argument. That purpose doesn't require God's existence either; it merely requires his belief in a set of principles that he attributes to God.
People have been promoting a variety of gods throughout history ranging from Thor to Zeus. Presumably he'll agree that Thor doesn't exist, but that presumably there were Scandinavians, way back when, who found purpose in promoting Thor's ideas. Will he claim that their lives lacked purpose because Thor wasn't real? Would they not have felt driven by purpose through their own belief? (I expect that he'll argue over this, but at least if you can agree on a definition of his purpose, you'll have something to work with.)