This is the first topic debated by Randal Rauser (a Christian) and John W. Loftus (an atheist) in their book God or Godless?
John Loftus concedes that if there is no God, then life has no intrinsic purpose and we have to either (a) muddle along in a purposeless existence, or (b) construct some kind of purpose for ourselves.
Yes, I agree with that. If it were proven to me that God did not exist, life would either become meaningless for me, or I would have to construct some kind of meaning. I’m pretty certain that, like most other atheists, I would follow the latter path. But it’s not as simple as that: like everyone else, I am shaped by my family background, education, culture, and previous experiences. The meaning that I constructed would be molded by all these things and, in my case, would still look something like a Christian meaning. But, given our Christian heritage here in the West, that’s true even of dyed-in-the-wool atheists. Even Richard Dawkins sings Christian hymns in the shower!
But there is nothing in John’s argument that suggests that a purpose-giving God does not in fact, exist.
Randal’s main point, I suppose, is this: the fact that most people think and act as if life had some kind of ‘deep purpose’ to life does provide evidence of the real existence of such purpose, and therefore to the existence of a purpose-giver. In my opinion, Randal stakes too much on this claim. I prefer to treat such considerations, not so much as evidence of God, but pointers to God.