How can 2 billion people be wrong? Surely I can't claim that 2 billion people are wrong about Christianity? Well I do.
First off, categorizing all these competing sects and factions as one religion doesn't really work. There are over 1.1 billion Catholics, 250 million Orthodox Christians, 7 million Jehovah's Witnesses, 12 million Mor(m)ons and about 590 million Protestants. Many of these religions claim that the other is wrong in a "you're going to hell" sort of wrong. Protestants demand salvation through grace whereas Catholicism requires participation in the seven sacraments.
Using the argument of "popularity"; a Protestant Christian would have to consider converting to Roman Catholicism, Sunni Islam (1.3 billion) or Hinduism (900 million) all of which claim more adherents than Protestantism. Truth cannot be subservient to popularity.
Using the argument of "popularity," at various points in human history Ra, The Jade Emperor, Zeus and Jupiter have all been real gods. Egypt, China, Greece and Rome have all had powerful civilizations far exceeding Israel's meager mark on history. Truth cannot be subservient to popularity
As a side note - using this argument there is no way Judaism could ever be considered as the true religion. Jehovah was a tribal god owned by a small, below-average civilization that never really makes anything of itself in the thousands of years that it exists.