If you don't believe in an all-powerful deity, you know that you're unlike most Democrats, just as you're unlike most Americans. But you also know that in the Democratic Party you'll find a lot of people who are like you and will welcome you. From the Republican Party, on the other hand, you get a message of unremitting hostility. Every time a bunch of dingbat Tea Party members of Congress gathers to say the Pledge of Allegiance and shout the "under God" part, every time Fox News ramps up the yearly installment of the War on Christmas, every time Repbulicans whine about how Christians are the real oppressed people if they can't get taxpayer money to prostelytize and force everyone else to listen to their sectarian prayers, non-religious folks get the message loud and clear: You're not one of us and we don't like you.
Hmm. First of all, let's not lose sight of a simple correlation thing here: young adults are largely responsible for the Nones' growth, and young adults are mostly Democrats. What's more, if I can join Waldman in wagering a guess, I'd blame the churches before the Republicans. It makes more sense to assume that left-leaning folks are leaving church rather than that young, nonreligious Republicans (already a smallish group of folks) are leaving the party because they just noticed it's been beholden to religious people for their entire life.
After all, if you're a liberalish 20-something who grew up Catholic, you've spent much of your adult life seeing your church's leadership prioritize such causes as keeping the sisters in line, opposing contraception, mandating more archaic language in worship and, of course, protecting child abusers. If you're part of the largest mainline Protestant denomination, your church still officially disagrees with your generational cohort about gays and lesbians' right to be affirmed in their full humanity. If you're an evangelical, there's a decent chance your social views have strayed somewhat leftward recently, and that your church hasn't kept up—not to mention the fact that your religious identity might have been less institutionally invested in the first place.
I'm not a 20-something but I was pretty recently, and I firmly believe there are wonderful, critical, life-giving reasons for left-leaning folks to stay in the church. But is there any question that there are also a lot of compelling reasons to leave?