Christmas ought to remind us what the whole thing is about.

By the “whole thing,” I mean Christianity, the enduring demands of Jesus and the remarkable fact that 2,000 years later, so many are still captivated by the birth of a baby in a stable.

In an increasingly secular time, it’s easy to diminish this holiday as a repurposed pagan celebration bathed in sentiment and commercialism. And I won’t pretend to be immune from the season’s sometimes syrupy sweetness.

I love seeing cars race down streets with Christmas trees tied to their roofs. I never tire of reruns of “It’s a Wonderful Life” — or, for that matter, Frosty and Rudolph. A regiment of nutcrackers musters every year in our front hall while a Nativity scene in our living room reminds us of the family that couldn’t find room at the inn.

But in the United States, we live in a moment when Christianity is losing ground, especially among the young. I speak not of demographic change (our country is more religiously diverse than ever, and honorably so) but of outright abandonment.

Americans under 40 are the least religiously engaged generation since we have been measuring such things. They are skeptical of organized religion, and many in their ranks see Christianity as hopelessly complicit with political figures they perceive as reactionary, authoritarian and intolerant.

Many Christians counter by pointing to voices that have been raised loudly on behalf of immigrants and refugees, the marginalized and the impoverished. (See, for example, Pope Francis.) But this speaks to the challenge Christianity faces: It stands before the world deeply divided. Its trumpet, in St. Paul’s metaphor, is giving an uncertain sound.

In one sense, there is nothing new here. Christians, like members of other faith traditions, have disagreed about worldly matters for centuries. Religion, as conservative thinker Irving Kristol shrewdly observed, can be both prophetic and rabbinic — on the one side, in revolt against things-as-they-are; on the other, insistent on observing the laws embedded in tradition.

You see this divide when progressive Christians, who cite the liberating Exodus story and the Sermon on the Mount’s personal and social demands, face off against conservative Christians who point to what Leviticus said about homosexuality and stress narratives of personal salvation (see those John 3:16 signs at football games). It’s no accident that the Gospel’s affirmations about the poor and the marginalized — along with Exodus and the prophets Micah, Amos and Isaiah — tend to be heard more often in African American churches than in conservative white congregations.

The skeptic would take all this as proof that religious faith, including Christianity, is a human invention that individuals and groups turn to their own worldly purposes. And Christians of all stripes often make the skeptics’ point by behaving in thoroughly un-Christian ways, perhaps especially in their arguments with each other.

Matthew’s Gospel reminds us of these words from Jesus: “You have heard that it was said, ‘You shall love your neighbor and hate your enemy.’ But I say to you, love your enemies and pray for those who persecute you, that you may be children of your heavenly Father.”

There isn’t a lot of that “love your enemies” thing going around, and I confess to finding it difficult myself these days.

But in the spirit of fellowship, I’d suggest that anyone who thinks of themselves as Christian must see this faith as something more than a venerable, comforting tradition — or as a ticket to heaven.

In his book “Simply Christian,” N.T. Wright, the former Anglican bishop of Durham, England, urges us to remember martyrs such as Dietrich Bonhoeffer, the theologian killed by the Nazis; Archbishop Óscar Romero, shot in El Salvador because he spoke out on behalf of the poor; and, of course, the Rev. Martin Luther King Jr.

“They are a reminder,” Wright wrote, “that the Christian faith still makes waves in the world, and that people are prepared to risk their lives out of the passion for justice which it sustains.”

Earlier this month, one of my theological heroes, Father Johann Baptist Metz, died at the age of 91. Metz insisted that “the salvation to which Christian hope is related is not simply or primarily the salvation of the individual . . . but as salvation of the covenant, of the people, of the many.”

If this were not true, I’m not sure Christianity would have flourished and survived into our time. At Christmas, we remember a child, but even more, we remember the sweeping hope his birth inspired.

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