Suppose we re-categorized Christianity as an adult activity. It would be like smoking, alcohol, voting, driving, sex, marriage, and (in some states) pot—things that you must be mature enough to handle wisely.
How long would this adults-only Christianity survive? My guess is that, starved of its primary source of new members, it would die out within a few generations.
We all have inside us what could be called a BS Detector—that common sense that helps us believe as many true things and reject as many false things as possible. For example, present most American adults with a case for Islam or Hinduism or Sikhism, and they will be extraordinarily unconvinced in the same way that claims for miracle cures, alien abduction stories, and great deals on swamp land in Florida would typically be rejected.
As adults, we’re far better at sifting truth from BS than we were as children. And that’s why Christians must be indoctrinated as children, before their BS Detectors are mature. This is the idea behind the Jesuit maxim, “Give me a child until the age of seven and I will give you the man.”
(The full version ends with “… but give me the man, and he will say, ‘Dude, are you insane? Who would believe that??’”)
Getting a 50-year-old who’s never smoked hooked on cigarettes is like getting a 50-year-old who’s never heard of Jesus hooked on Christianity. It’s possible in both cases, but it’s far easier when you make the appeal early in life.
Imagine this conversation between the father of a 6-year-old child and the grandmother.

Grandma: “Little Johnny is old enough for me to take to Sunday School now.”
Dad: “You can take him when he’s 18, but I’d prefer he stay out of church until then.”
Grandma: “But 18 is too late! By then he’ll be set in his ways. He won’t accept the truth then.”

What kind of “truth” is it that must be taught before people are mature, before their BS Detectors are fully functioning? Grandma realizes that only before someone’s BS Detector is operating correctly can the beliefs of religion be put into someone’s head. This is a very poor stand-in for truth.
Many Christians will agree that Christianity needs access to immature minds to survive. But what does this say about the evidence behind the Christian claim that God exists?

The first principle is that you must not fool yourself—
and you are the easiest person to fool.
— Richard Feynman

(This is a modified version of a post originally posted 9/8/11)