"2 million people think it is true!"

 

"All of the Christians can't be wrong!"

 

This relies on the idea that if a large number of people believe something to be true, then it must be true. This is clearly not the case. The truth of a claim in no way depends on how many people believe it. At one point, almost everyone in the world believed the Earth was flat. According to this argument, the Earth actually was flat. Which of course, it was not. Huge numbers of people can be wrong, and can be wrong about the same issue. 

 

When it comes to religion, the problem is even more obvious. You have Christians saying their religion must be right because millions of people can't be wrong. And you have Muslims saying exactly the same thing. But they cannot both be correct.