The argumentum ad populum is, generally, a logical fallacy which holds that, because many people believe a thing to be true, the level of professed belief itself demonstrates the truth of it. The argument has traditionally been challenged on its face with the fact that many widely held views turned out to be false -- the geocentric theory of the planets, for example, or the belief in sneezes being triggered by demons trying to enter the body. And yet, the argument is actually valid in some situations, such as the expression of social customs which are effectively popular beliefs themselves -- for example, that in America in the year 2009 it is acceptable for a man to kiss a woman in public. When applied to religion, the argument has been applied on two levels, one being that the broad practice of religions in general proves that there's something there underlying the proclivity of mankind to believe in anything at all; and more narrowly that the size or growth rate of particular religions indicates the exclusive truth of any of those particular religions.
The more general application suggests an argument for the common belief in religious explanations flowing from a collective subconscious experience of some truth of that nature. But the narrower application is unsustainable in the face of logic and experience, not simply because more specific collective beliefs can just be factually wrong, and not simply because equally large groups of people swear by differing mutually exclusive belief systems. The biggest problem in this argument for such belief is this: the belief professed simply does not match up with the actions of those who proclaim it.
The Evidence of the Lack of the Faith Proclaimed
"Four billion people say they believe in God, but few genuinely believe. If people believed in God, they would live every minute of their lives in support of that belief. Rich people would give their wealth to the needy. Everyone would be frantic to determine which religion was the true one. No one could be comfortable in the thought that they might have picked the wrong religion and blundered into eternal damnation, or bad reincarnation, or some other unthinkable consequence. People would dedicate their lives to converting others to their religions."
Adams continues with the clear contrast of reality to this hypothesis:
"A belief in God would demand one hundred percent obsessive devotion, influencing every waking moment of this brief life on earth. But your four billion so-called believers do not live their lives in that fashion, except for a few. The majority believe in the usefulness of their beliefs—an earthly and practical utility—but they do not believe in the underlying reality."
And finally, in more concrete terms, Adams analogizes:
"If you believe a truck is coming toward you, you will jump out of the way. That is belief in the reality of the truck. If you tell people you fear the truck but do nothing to get out of the way, that is not belief in the truck."
But Adams barely scratches the surface of what is going on in this analogy, even. He speaks of those who claim to fear an oncoming truck and yet take no steps to "get out of its way." But many people who claim to be religious go much further than that. They take, it seems, excessive opportunities to run from the safety of the roadside into the path of the truck. And in particular they throw themselves in front of the truck in an effort to punish those who are already standing in its path and professing not to believe in it.
Consider this. Someone that you claim as a moral enemy is standing in the middle of the road, someone who you think is a bad or even an evil person because they embrace a set of values contrary to your own. You claim to believe a truck is barreling toward that person to impose judgement on their behavior -- an invisible truck, but a powerful and deadly one nonetheless. If you truly believe in the presence of that truck, you would stay off that road, but the action of many who profess such a belief is instead to do the equivalent of grabbing a club or other weapon and running headlong out into that road so they may attack their perceived enemy. Never mind that this puts you in the path of the truck as well. It is the fact that you feel the need to personally take action to harm this enemy that suffices to demonstrate that you are not genuine in your belief that the oncoming truck will harm him.
This analogy plays out in real life in courthouses and statehouses and other legislative and governing bodies across the world, where lip service believers seek to align the coercive power of the state against behavior which they proclaim to be disfavored by their deity -- an unnecessary measure if the deity itself was truly already expected to punish that behavior. In short, people of many religious backgrounds claim to believe in a God that sets forth laws and punishes transgressors of those very laws, and yet they almost universally act as if this is not so in their effort to get other, decidedly secular entities to punish those same asserted transgressors. Many people proclaim the belief that God imposes eternal punishment on the condemned, and yet those same people seek to have the state impose, on behalf of their religion, the execution of those very condemned -- a fruitless and wasteful measure in the face of eternal condemnation.
The overall human tendency to believe in a spiritual underpinning of the Universe can be taken as evidence in favor of the proposition that such an underpinning exists. Not conclusive evidence by any means, and susceptible to alternate explanations, but evidence nonetheless. But the vast variety of particular beliefs in itself weighs as evidence against the particular truth of any one of those particular beliefs.
The multitude of particularized beliefs could be read as an indication that there are multiple truths, in other words a multitude of contradictory Gods (or God-like entities), or that one God manifests itself in a multitude of contradictory forms. But if belief is a subconscious manifestation of a higher truth, then the existence of many such entities would more likely permeate our beliefs, and our tendency would be towards polytheism or polydeism; and for one omnitheistic entity to manifest itself in many different ways, giving contradictory and even mutually exclusive directives to its followers and putting them on a course for conflict with each other, that entity would have to be intentionally cruel or at least highly irrational to the point of insanity.
What the multitude of beliefs is evidence of, then, is that such a spiritual underpinning exists, but is dormant, and gives no conscious guidance whatsoever as to its nature. Thus, man is left to take this deep-seeded sense of the nature of the Universe and paint upon it his own biases, prejudices, hopes, and fears. The tendency of humans to proclaim belief in a judgemental higher power, and yet to seek to carry out for themselves the judgements they would attribute to it, speaks more directly to the truth of this condition than the sum of all myriad professions of particularized belief.