Brad Pitt had a line in 12 Monkeys that resembled this idea. Something like "There's no right and wrong, only popular opinion." He cited the fact that 100 years ago, no one knew anything about germs, and no one cared... but now, companies make all kinds of money by selling anti-bacterial soap, so they clearly just made it up.

This is usually called relativism, the idea that some things are true because a certain group of people believe them to be true. If a bunch of scientists believe something's true, most people will take their word for it until another scientist disproves it. If a religious leader or text says something's a fact, fundamentalists will form picket lines against or murder people who claim it's not a fact, and other religious followers will usually accept it as fact no matter what anyone else says.