This is something you hear very often from believers. I have touched on it before. It is a logical fallacy known as the argumentum ad popularum or argument by popularity. The reason it is a fallacy is that the truth of an assertion is not determined by it's popularity; it is determined by objective facts. A good first response is to simply point that out:
"A lot of people used to believe that the Earth was flat, but that didn't make it true."
"Whether something is true doesn't depend on the number of people who believe."
"Facts don't depend on how many people believe in them."
There are a couple of other points to keep in mind here. This "argument" is a reflection of the religious mindset, which, frankly, seems to be the same as the herd mentality. Most religious people are constantly focused on what others think, because they are entirely concerned with their place within the group. They think that having other people agree with them proves them right because all that matters to them is what the rest of the group thinks of them. This is, of course, an obviously false assumption that merely proves how little confidence they have in their own judgment. Sometimes it might help to point this out.
"You think that what others think is always what you should think? Do you really have so little confidence in your own intelligence?"
There is also, however, a whiff of threat in this argument. In many cases, there is a unstated message: "We are many, you are one. You can't win because our might makes us right." Might makes right is a recurring theme in the thought of the religious and pointing it out will work on occasion, but this is probably not one of them. I mention it here because it may be that the person is making a subtle threat. Often believers retaliate by trying to make non-believers unpopular, which can have far reaching effects on one's life. If you think this is the case when you hear the "many people believe" point, then you should not push your arguments any further and be very careful how you treat this person in the future. In fact, you should get away from this person, but, if possible, I recommend buttering the person up a bit before you go.
Remember, believers are not ruled by rationality, they are rationalizers. If you leave them feeling insulted and angry, they will stew about it and start to find all sorts of things wrong with you. One of the better things to say is
"I don't think you need other people to tell you what to think. I think you are smart enough to figure this out on your own."
I recommend this not only to smooth the believer's feathers but also because I believe it is literally true. I think almost anyone is smart enough to figure out that religion is nonsense if allowed to think about it on their own. The trouble is that they are not allowed to do that. But, that is the subject of another post.