Rothbard knows the libertarian community as well as anyone. If he thinks most libertarians are atheists, I’ll take his word for it. Ayn Rand was an militant atheist, and millions of libertarians take her words as gospel. Especially the hormone-addled childless youth who live and vote like atheists.
Here’s an obvious truth, like water flows downhill. People grow up with religious and philosophical beliefs first, which ultimately shape political beliefs. They learn about God at age four, start walking precincts with their parents at ten, and voting at 18. What’s so controversial about that?
He knows the libertarians he's met or conversed with - there's no evidence on the table that he's ever attempted to survey a representative cross-section of the entire libertarian community, or claimed to have done so.
People grow up with religious and philosophical beliefs first, which ultimately shape political beliefs.
And as the link I've posted shows, one can grow up with Christian beliefs and end up with a political philosophy of minimal government:
"NO KING BUT JESUS: Romans 13 and the Perfect Law of Liberty" - http://web.archive.org/web/20071015041700/http://www.freerepublic.com/forum/a3ad0b80b3a17.htm