Here's Jan's argument in a nutshell:
This argument is given in the face of a world in which, in fact, millions of people do not believe that God exists. That is, a world where certain people not only can conceive of a world where God does not exist, but who honestly believe that is the true nature of the world.
Jan's argument is that people who actually do imagine a world in which God doesn't exist, and believe in the truth of that world, are wrong because they don't know what God really is. That is, Jan thinks that atheists and so on are denying the true definition of God - Jan's definition - and that if they really accepted what God actually is - i.e. accepted Jan's conception of God - then they'd have no choice but to believe in God.
To put it in simpler terms, Jan would like atheists to accept that his God is a logical necessity because Jan's universe needs a God-like creator or else his house of cards falls down. The problem is that for his God to be a logical necessity we all first need to accept that his definition of God as the Creator etc. is the only possible conception of God, and that God is essentially synonymous with the universe. Therefore, no God = no universe. Apparently Jan is stuck in his own little box here and can't see out.
Is it really so hard to imagine a world in which living things didn't breathe? It seems clear to me that such a world would be very different from this one, not the same, but such a world is conceivable. I'm not sure what the issue is with imagining such a thing.