It’s the old “50 million Elvis fans can’t be wrong” argument: lots and lots of people believe in God; therefore, God must exist. The Prosblogion takes a look at this argument:
The proposition that the mere popularity of a belief might constitute evidence for its truth may strike us as odd. Mill, for instance, argued that common opinion might be OK for the common folk who are unable or don’t feel entitled to form their own opinion, but to us, thinkers “the argument from other people’s opinions has little weight. It is but second-hand evidence; and merely admonishes us to look out for and weigh the reasons on which this conviction of mankind or of wise men was founded”.
Kelly regards this Millian view as implausible, especially in the light of recent social epistemology (see above).As for common consent, he observes is often reasonable to defer to majority opinion. An example he offers (and I encountered in real life many times) is the following: suppose I think I need to put out my bin on Friday as always, but I notice everyone else’s bin standing outside on Thursday. I will, justifiably, revise my beliefs and put my bin out too (I usually only afterwards realize there’s a bank holiday upcoming or some such, but by then I have already deferred to the majority view). Social epistemologists recognize this importance of other people’s beliefs as prima facie evidence for the truth of these beliefs.
There’s lots more, including lots of brain-aching logic stuff. Even if you don’t understand it, reading it makes you feel smarter.