DURING his in-flight interview between Cuba and the United States, Pope Francis bristled a little when asked about people who claim he’s too liberal, left-wing, even Communist. In his comments on economics, immigration and the environment, he insisted, “I’m sure that I haven’t said anything more than what’s written in the social doctrine of the church.”

The pontiff has a point, but so did his questioner. Yes, Catholic social teaching does not fit the normal categories of American politics, and popes always come to our shores bearing critiques of right and left alike. But every pope has different interests, a different set of points to stress --- and Pope Francis’ message, whether on-script or off-the-cuff, is particularly distinctive.

He is certainly not a Marxist, and he’s not a “liberal” as American politics understands the terms. But he has been a gift to liberals who are also Christians, to religious believers whose politics lean left.

It’s a gift the religious left sorely needed, because the last few decades have made a marriage of Christian faith and liberal politics seem doomed to eventual divorce. Since the 1970s, the mainline Protestant denominations associated with progressive politics have experienced a steep decline in membership and influence, while American liberalism has become more secular and anti-clerical, culminating in the Obama White House’s battles with Francis’ own church. In the intellectual arena, religiously-inclined liberals have pined for a Reinhold Niebuhr without producing one, and the conservative fear that liberal theology inevitably empties religion of real power has found all-too-frequent vindication.

Pope Francis has not solved any of these problems. But his pontificate has nonetheless given the religious left a new lease on life. He has offered encouragement to Catholic progressives by modestly soft-pedaling the issues dividing his church from today’s liberalism . abortion and same-sex marriage . while elevating other causes and concerns. His personnel decisions have confirmed that encouragement; his rhetoric has reinvigorated left-leaning Catholic punditry and thought. And his media stardom has offered provisional evidence for a proposition dear to liberal-Christian hearts . namely, that a public Christianity free from entanglements with right-wing politics could tug the disaffected back toward faith.

The pope’s address to Congress last week was an illustrative moment. The issues that have bound many American Catholics to the Republican Party were mentioned, but obliquely; the word “abortion” was not used, the threats to marriage were not identified. But on the environment, immigration, and the death penalty, he was much more specific and direct. And of the figures he invoked from American history, three . Martin Luther King, Thomas Merton and Dorothy Day . belonged to the midcentury golden age of Catholic liberalism, an era when progressive politics and Christian faith seemed to make a lovely harmony.

You could interpret that invocation as a challenge to liberal-leaning Christians, an invitation to find such a synthesis again. And believers whose politics are further to the right, this columnist included, should actually want that challenge to be met. A revitalized religious left, a Christianity that doesn’t feel like the province of a single political faction, would be a sign of religious vitality writ large. And I would far rather debate politics with Cornel West or the editors of Commonweal than with a liberalism that thinks it can impose meaning on a cosmos whose sound and fury signifies nothing on its own.

But there are deep reasons why liberal Christianity has struggled lately, which a Francis-inspired revival would need to overcome. One is the tendency for a liberal-leaning faith to simply become a secularized faith, obsessed with political utopias and embarrassed by supernatural hopes, until the very point of churchgoing gradually evaporates. (It’s not a coincidence that the most resilient of left-leaning religious communities, the African-American church, is also the most frankly supernaturalist.)

The other is religious liberalism’s urge to follow secular liberalism in embracing the sexual revolution and all its works . a move that promises renewal but rarely delivers, because it sells out far too much of scripture and tradition along the way.

The first tendency is one that this pope’s example effectively rebukes. However “left” his political impulses may be, they are joined to a prayerful and devotional sensibility, an earthy, Satan-invoking zeal that has nothing arid or secularized about it.

The second tendency, though, is one that Francis has tacitly encouraged, by empowering clerics and theologians who seem to believe that Rome’s future lies in imitating the moribund Episcopal Church’s approach to sex, marriage and divorce.

How far to go with them is the question that awaits the pope in Rome this fall, and that hangs over the springtime for liberal Christianity his pontificate has nurtured.

How it’s answered, and what follows, will determine whether we’re watching something genuinely new and fresh emerge . or whether, after the cheering ends, the same winter that enveloped liberal Protestantism after the 1960s will claim Franciscan Catholicism as well.