So sheer quantity of believers doesn't work for making a point.
They're basically saying that sheer quantity of believers in anything proves that their God exists!
Coming from a Christian perspective however, I would agree in general people have evidence to believe in God. I don't intend the quote from the Bible below to serve as an sort of evidence. This would not be a logical line of reasoning for someone who does not believe in the truth of the Bible. However, it may serve to further clarify my position.
"For since the creation of the world God’s invisible qualities—his eternal power and divine nature—have been clearly seen, being understood from what has been made, so that people are without excuse." -- Romans 1:20
It definitely doesn't provide any proof, but I didn't say that. I said "sheer quantity of believers doesn't work for making a point", as in, how can the number of people in any way support the validity of any beliefs held by those people?
There's plenty of concrete historical evidence that beliefs held by many many people turned out to be wrong. It's basically the story of science. I.e. we have clear evidence that the fact that a lot of people hold a belief does not mean anything about the truth of that belief.
> In any case, at least Christianity and Islam each have over one billion believers globally.
But that illustrates my original point.