The argument goes something like this:
Billions of people agreeing on something are likely, prima facia, to be right.
Billions of people agree on there being a God.
Therefore, it is likely, prima facia, that there is a God.
This is a simple argument and it has a kind of plausibility to it, but it can easily be mishandled and its force lost on listeners. The biggest of these problems being the ad populum fallacy.
The Ad populum fallacy, lit.: “to popularity”, refers to the belief that something is true because it’s popular. Millions of americans believed that slavery was ethically permissible, they were wrong. Millions of people once believed the earth was the center of the universe. They too were wrong. Truth is not a popularity contest or a democracy. The whole world could be wrong, so mere popularity is not itself a strict proof of something’s truth.
But neither does the mere possibility of their being wrong PROVE that they are likely wrong. Possibility is a long way from probability and likelihood. Millions and billions of people could be wrong, but they have a pretty good record in other regards that should not be forgotten. Most people agree that they themselves exist, that the extra-mental world exists, that their senses tell them about the world, that numbers accurately describe reality, that emotions happen, that cats can be domesticated, that mosquitoes cannot, that bicycles have 2 wheels, that cameras take pictures, that science informs us about the world, that reason and thinking occur, and that other minds exist, that sugar tastes sweet, that salt tastes salty, etc.. It would be cumbersome, to say the least, to demand that people PROVE all of these widely held beliefs before the skeptic steps down from his skeptical throne to kiss the ring of dogma. The reasonable person could just as well operate on the assumption that when people widely agree on a given idea, that idea is more likely true than not. The “Billions-of-believers” argument should be wielded as a credibility argument and not as a strict veracity argument. The idea merits credibility since it is unlikely that billions of people are deceived so wildly as to not even be partially true about God’s existence.
Treated correctly, the theist can assert this argument even while admitting that this line of evidence can be overturned with greater and sufficient counter-evidence. The atheist needs to show that these billions of people are likely wrong or otherwise non-credible.
Furthermore, the skeptic is not necessarily rational in the absence of sufficient reasons for their skepticism. Doubt is just as liable to be lazy non-commitment as it is to be tactful self-defense. There is nothing easier than to doubt. Sure people can exercise faith in lazy-minded ways too. But there is nothing innately wise, or rightly guided about doubt in general. Only responsible and rightly directed doubt is wise. One can easily be irrational in one’s doubts so it is not at all obvious that skepticism is automatically warranted wherever billions of people agree on something. That methodology seems to be systematically arrogant, presuming to be justified in disbelief without having to give any justifications for disbelief–even if a claim is plainly obvious, widely held, probably true, or otherwise staring the skeptic in the face.
Lastly, the skeptic still needs to offer a better alternative explanation for why there would be such widely held god-belief among such different people over such different geography having such different reasons and causes for their belief. Typical explanations tend to be dismissive, and are easily countered with exceptions. “Wishful thinking”–but people don’t “wish” that non-believing family members would die lost/going to hell. “Cradle-faith”–but many people convert to faith late in life, and many abandon the faith of their upbringing. “No direct empirical evidence”–but that can only be expected of a material God, and no world religion affirms that at their core. “Subjective experience is unreliable”–But vast intersubjectivity on the order of billions of people borders on conspiracy theory.
And that last point is what the Atheist rebuttal comes down to, a conspiracy theory. There is a vast systematic delusion at work in the world playing off of people’s weak psychology, wishful thinking, and habit of gullibility. It is only the enlightened atheist who knows the real truth, that all these billions of people are deluded. Okay, interesting theory. Now prove it. Your skepticism leaves me skeptical. I don’t trust your doubting. Show that it is wholly unjustified to treat the wide agreement of billions of people with not the slightest modicum of initial credibility.