Well the whole world wouldnt believe something like that. Hell, somewhere in the world, someone may believe that the world is flat. The whole world is never going to agree on anything. But as for the millions of people believeing one god... The whole world believes lots of different religions. Christianity is only as big as bhuddism, islam, and other large religions. In fact, those religions have been around longer than christianity. And even if the entire world was christian... Choosing your religion based on what everybody else believes is dumb. You are supposed to find the answers yourself, not to follow the croud.
Many kids don't get a chance to choose for themselves because their parents jam their beliefs into them before they can even understand it.
I don't have a problem with people believing in a god or the afterlife but if they ever use that as an excuse to commit atrocities, such as the crusades or 9/11 or even a single murder or attack, that's when it crosses the line.
IMHO God(Christian God), the alleged creator of the universe, bearer of all knowledge, creator of mankind, would not be so vain as to require that a person need only believe in him/her in order to get into heaven.
"For God so loved the world that he gave his one and only Son,[a] that whoever believes in him shall not perish but have eternal life." - John 3:16
Going by this criteria Hitler has a better chance of getting into heaven than I do.
The concept of heaven was created by man in order to ease the fear of death.