Almost everyone has some implicit idea of the marketplace of ideas. It is the notion that ideas will be examined by everyone or almost everyone, arguments for and against ideas will be made by many people, and that eventually the best ideas will come to the fore and be accepted by larger and larger numbers of people. It is referred to, metaphorically, as a marketplace because it is comparable to a marketplace with a number of people trying to sell their ideas and the fact that everyone is capable of “buying” an idea by adopting it.
The underlying assumption is that a good idea will eventually be recognized as such and that it will come to dominate much like a superior product will eventually become the most popular product. Good ideas can withstand criticism; bad ideas can’t. The multiple participants through their contributions of new arguments and ideas, as well as the repetition of those arguments and ideas they find compelling, and their adoption of new ideas they find compelling drive this metaphorical marketplace and make it work.
This marketplace has to have certain conditions to function properly, however, just like any other marketplace. Fraud and restraints on trade prevent the marketplace from functioning properly. Without free, open and honest trade, the marketplace cannot render an appropriate and accurate judgment.
The religious know of this marketplace and frequently make implicit reference to it. They say “a lot of people believe”, or words to that effect. Usually this is meant as an argument that their beliefs are valid; sometimes it is meant as an implicit threat of mobbing to those who dare to dissent. As an argument that their beliefs are valid it fails miserably. This is because the particular marketplace they mean has been tampered with extensively and doesn't work properly and because, even when it does work properly it doesn't always reach the correct conclusion.
The number of people who accept a proposition does not prove that it is true. This is a logical fallacy known as the argumentum ad popularum, or argument from popularity. The number of people who believe something is proof only of their conviction by some means, not of the proposition itself as the history of the flat earth theory and the Ptolemiac earth centered universe model make clear. At one time nearly everyone believed the earth was flat—that didn’t prove it was flat by one jot. At one time, nearly everyone believed that the earth was the center of the universe—that didn’t make that hypothesis true, either.
A quick way to make this point is simply to say:
"The number of people who believe something doesn't prove it's true. Once upon a time most people thought the world was flat."
I recommend making both of these statements. The second one has more emotional punch, but I find that the religious simply don't understand the implicit logic of it unless you make it explicit. And, you may have to repeat yourself and make it explicit several times before the point sinks in. My personal experience bears this out. In addition, a recent study found that, in fact, the ability to absorb and process new information was an even greater predictor of non-belief than intelligence.
The marketplace of ideas is only as good as the information available and the judgment of its participants—as is true of all marketplaces. With regard to the issue of belief, however, there has been a deliberate, long-standing effort to undermine both the quality of the information available and the judgment of the participants in the marketplace.
The religious have a long history of burning and banning books, as well as authors, that dare to disagree with them. At one time they even burned people at the stake for translating the bible into local languages. They have burned entire libraries and executed people for disagreeing on very minor points of doctrine. Today, such practices can't be used in most countries. Though, apparently, it is still possible to ban entire schools of thought in Muslim countries. I have no doubt many of the religious in the U.S. would like to return to such practices. In countries, where the religious hold power, blasphemy laws are enacted to force dissenters to be silent. If that is not possible (as it isn't in the U.S.--for now) the religious must instead rely on more subtle techniques, such as bullying, discrimination, lying, slandering, bribing, blackmailing, etc., not to mention the incredible power of childhood brainwashing followed by weekly group reinforcement sessions.
Consequently, another response can be used, perhaps as a follow up to the first:
"The number of people who believe isn't even meaningful because of the extent to which you religious types have deliberately prevented the proper functioning of the marketplace of ideas."
"The number of people who believe isn't even meaningful because of the extent to which you religious types have deliberately prevented people from looking at the issue fairly and objectively."
Then you can mention the most obvious dishonest tactics they use such as childhood brainwashing, bullying and discrimination, and explain why each of them is not only wrong but an implicit admission that religion is nonsense--good ideas don't need that sort of help.