Believer asks: How do you explain the popularity of Christianity?
Skeptic answers: Well, I say that since people want to go to heaven, they turn to Christianity (or to a different religion) and its promise of heaven. Ironically, popularity of any Christian church doesn't guarantee that it was the original Christian church. But it's popular because it's so-called cafeteria Christianity, a version or other of Christianity.
Islam and Mormonism are popular, too, but certainly not because they're both right. Rather, because they, too, teach the doctrine that their adherents will go to heaven.
Recap: Why is Christianity so popular? Because folks wish to go to heaven and believe that Christianity will get them there. But I say there's no assurance the doctrine is "spiritually" true.