They can look at the evidence for each and decide if there are good reasons to choose one faith over the others. They can debate those with different cultural and philosophical perspectives and see if, by putting on someone else's shoes, they can better understand why others come to very different conclusions about religion and God(s).
It seems to me, if Christianity can stand up to this critical examination at the hands of the "everyman" that information technology is now facilitating, then America will continue to be religious. If it can't, America will become more secular. Either way, reason (which is responsible for an incalculable amount of good in our society) will have won out. It will mean America has chosen what to trust wisely, and that will be a good thing.
Anyhow, you present a false dichotomy. Reason is not what separates. What separates is a preference for the body over the soul and an temporal life over an eternal life. You think the ancients didn't trust in reason.