Christmas,” Swiss theologian Hans Urs Von Balthasar wrote, “is not an event within history but is rather the invasion of time by eternity.” By this he meant that the Christmas event was not limited to a particular moment or even an epoch, but signaled an unfolding apart from the limitations of time. The improbability of eternity disrupting time itself is the principal upending in the long list of unexpected reversals that characterize the Christmas story.

Startling events set that story in motion: a young woman of no special social standing is visited by an angel, and in short order the virgin is pregnant. Her betrothed, who, according to custom and religious law, has every right to send her away or have her executed, instead goes through with the marriage. Underneath a star so bright it is visible in daylight, the couple travels to another city and finds not a single room available for the mother of the Son of God. Thus, the Messiah is born and laid in a manger, a trough reserved for animal feed.

It’s very strange, a series of incongruities. Underscoring all of them is the notion that God would want anything to do with humanity. This, Søren Kierkegaard writes, is the core absurdity of Christianity itself:

Christianity teaches that this individual human being — and thus every single individual human being, no matter whether man, woman, servant girl, cabinet minister, merchant, barber, student, or whatever . . . exists before God, may speak with God any time he wants to, assured of being heard by Him — in short, this person is invited to live on the most intimate terms with God! Furthermore, . . . for this very person’s sake, God comes to the world, allows himself to be born, to suffer, to die, and this suffering God — he almost implores and beseeches this person to accept the help that is offered to him! Truly, if there is anything to lose one’s mind over, this is it.

Kierkegaard is right: there is a note of madness in the idea that, for so many average folks and motes of dust in sunbeams, God — the creator of the universe, infinite, and omnipotent — would submit to human flesh and an earthly life. In that sense, Christmas is the introduction to a totally astonishing plan.

And yet too often, Christian thought is sterilized and diluted until it resembles little more than popular wisdom, or worse, common sense. “The sum total of all human wisdom is this ‘golden’ (perhaps it is more correct to say ‘plated’) mean,” Kierkegaard writes, “nothing in excess. Too little and too much spoil everything. This is bandied about among men as wisdom, is honored with admiration. . . . But Christianity makes an enormous giant stride beyond this nothing in excess into the absurd; that is where Christianity begins . . .”

Christmas is where Christianity begins, and, as Kierkegaard observes, it is rife with the strange and unexpected. Optimally, then, it should serve Christians as a time to mine tradition and practice not for their most tired applications, but for those that are unexpected and those that lead us in our pursuit of the unexpected.

There is, after all, something revolutionary in Christianity — a tendency to upend, reverse, and radically transform. In Mary’s magnificat, the song of praise, she offers at her meeting with her cousin Elizabeth, she proclaims, “My soul magnifies the Lord, and my spirit rejoices in God my Savior, for he has looked with favor on the lowliness of his servant . . . He has brought down the powerful from their thrones, and lifted up the lowly; he has filled the hungry with good things, and sent the rich away empty.” This list of upsets issues from the mouth of a peasant girl who has been promoted to an almost unimaginable status. That the radical reversals of Christmas are enumerated to us by a young woman of no particular social standing is itself an incredible bit of turnabout.

The revolutionary character of Christianity is usually washed out and mostly confined to specific political moments when it’s useful to refer to it. But this selectivity, too, should be upended. Christianity is at all times concerned with the poorest, the most vulnerable, the most oppressed; it is permanently interested in reversing this order, in aiming at and accomplishing the unexpected. Christmas, the moment when time is invaded by eternity, is the moment when the reversal of all oppression becomes not possible but necessary. The unlikeliest upsets of order become, in the moment of Christmas, the beginning of Christianity itself, and remain essential to its character.

There is no Christianity, therefore, that is not revolutionary. It is possible to construe Christmas as another one of those soothingly cozy Christian celebrations, but it is more accurate to construe it as a call to revolution. From this moment on, nothing of the old order can be left intact: Christ has come to uplift the poor and bruised, and his example is Christianity’s command.