Anyone who proposes that a religion "has to be true because if it was a lie it would have died out" only needs to consider, let's see, the continued persistence of Scientology, Moonies, Jehovah's Witnesses, Rastafarianism, Appalachian snake-handling cults, Mormons, Vodun, and on and on.
These were all clearly made up by people; the social context and historical antecedents of each are laid out for all to see on the internet. Yet they remain and keep gaining converts every day.
The 5000 year old polytheistic religion of Hinduism, practiced today by nearly a billion people, many of whom are willing to die for their faith, should be enough evidence that being untrue doesn't stop a religion.
Why would Christianity be any different?