One popular argument against God's existence is the divine hiddenness argument. The basic argument goes something like this: If God exists, he'd do everything he reasonably could to make as many people believe in him. Since that hasn't happened, God doesn't exist.
There are many sophisticated formulations and variations on this basic argument, as well as many sophisticated counterarguments–which reflect the varied theological commitments of the philosophers in question. Here's a more detailed version:
The second version starts with a more particular premise concerning the God described by the New Testament, especially on the evangelical Christian interpretation of that text.3 According to this version of ADH, the (evangelically interpreted) New Testament makes it clear that God wants all of God’s human creatures to believe the truth of ‘the gospel message’, one of whose crucial elements is that ‘[t]he ruler of the universe sent his son to be the savior of humanity’.4 The particularity of that initial premise allows the second version to go more quickly than the first: the God described by evangelical Christianity would see to it that all cognitively and affectively capable human beings believed the gospel message. Yet only a minority of all cognitively capable human beings have ever believed the gospel message, including the claim that the ruler of the universe sent his son to be the saviour of humanity. So no God of the kind described by evangelical Christianity exists.
Contemporary demographic data illustrate the lopsided distribution of theistic belief. The populace of Saudi Arabia is at least 95 per cent Muslim and therefore at least 95 per cent theistic, while the populace of Thailand is 95 per cent Buddhist and therefore at most 5 per cent theistic. The approximate total populations are 26 million for Saudi Arabia and 65 million for Thailand.
Why on earth (literally) should the territory of Thailand harbour a high proportion of souls predestined for damnation and that of Saudi Arabia or (better, for Calvin) post-Reformation Europe a much smaller proportion?
But even if one concedes the value of the world’s religious diversity, response (6) does nothing to explain why this diversity manifests itself so often in clusters of believers, many of which exist in isolation from one another; why doesn’t this valuable diversity flourish within the cultures of Saudi Arabia and Thailand? Theistic explanations must account for this geographic patchiness in terms of reasons God might have for allowing it, and such reasons seem hard to find.
One response is that God doesn't make himself more evident to more people to avoid permanent rebuff from immature believers who might become resentful over evils they or their loved ones are made to suffer and blame God (Travis Dumsday). However, that fails to explain why God allows them to die in unbelief.
i) I think the divine hiddenness objection is a powerful argument against freewill theism. It's trivially easy to think of examples by which God could lead more people to believe in him. So the very fact that we resort to theistic proofs undercuts freewill theism. Theistic proofs would be unnecessary if God directly manifested himself to more people.
ii) The hiddenness argument lacks the same traction when it comes to Calvinism. Calvinism denies a key premise of the argument. God never wanted everyone to believe in him. So the fact that there are many unbelievers is consistent with Calvinism. Indeed, that's an implication of Calvinism, given reprobation.
iii) Still, that, of itself, doesn't explain the demographic disparities. What about that?
To begin with, Maitzen's comparison between Muslims and Buddhists is theologically clueless. From an eschatological standpoint, Muslims are no better off than Buddhists. Believing in a false god is no improvement over believing in no god. Idolatry is no better than atheism. Both Muslims and Buddhists are hellbound.
iv) To suggest, as Maitzen does, that Buddhists are atheists is simplistic. Folk Buddhism is not atheistic. And folk Buddhism is more demographically representative than philosophical Buddhism.
v) More to the point, we need to consider the demographic distribution in time as well as place. Over the centuries, there's been an exponential growth in human population:
It wasn't until around AD 1800 that the total population crossed the 1 billion threshold. And it's currently about 7 billion. So historically unreached people-groups could make up for lost time in a hurry. That's because there's a far larger percentage of humans living in the recent past, present, and projected future. Hence, Africans, Indians, and Asians could overtake Caucasians in the sum total of Christian adherents. Given the rapid acceleration in population growth, it takes less time than you might imagine for unreached people-groups to catch up with 2000 years of church history, and surpass the northern hemisphere. Cumulative totals must take time and well as place into account.