Catholic teachings

I heard a very interesting rumor yesterday while I was at work. I'm not sure if it's true, but I thought I would put it out there. Two girls and I were somewhat talking about birth control, for about 2 minutes, right before we started calling people again, and one of them said there is a Vatican document or something like that (I'm assuming she meant an encyclical or something) that says if the majority of Catholics don't believe something or don't practice it, it becomes null. Is that true? I find it slightly hard to believe. She claimed that somewhere close to 85% of Catholics (and then thought that number might only be American Catholics, who are somewhat different from other Catholics in some ways) don't believe in birth control, using this to say it shouldn't be a Catholic teaching anymore. What are your thoughts on this? And is there really church teaching that if a majority stops believing/practicing it, it's not a teaching anymore? I seriously find that difficult to believe-if the majority of Catholics stopped believing in the Eucharist, it would still be a teaching. Same with many other things that our church has been based on.