It all started when a black girl walked into church.

That’s not a normal sight in our 800-year-old monastery in Krakow, Poland.

This young lady was wearing a calico dress with little Jesuses printed all over it, rays of light gushing like streams from their tiny hearts. That is what Catholics know as the image of Divine Mercy. Maybe she got it in Dakar; it’s not a pattern you see much at H&M.

It was the right dress for the moment. Because once that lady clacked in her bedazzled sandals over our basilica’s chessboard floors, there came a river: a thousand, then ten thousand, then twenty thousand behind her.

Like a Walmart greeter, I waded there in midstream.

I spent the week from July 25 through July 30 standing at the door of our big gothic church. The humid weather was made downright swampy as the respirations of the crowd filled the space. I wore nothing but a habit with a pair of hiking shorts underneath — and my own sandals, which are stamped “Nimrod.”

I kept smiling.

I was placed there by my superior because, like certain Arab merchants, I can guess travelers’ origins pretty accurately and greet people in many tongues. I passed out brochures in seven languages. “Benvenidos” “Willlkommen” “Salut, tout le monde,” I said.

“Posso aiutarvi a trovare qualcosa?”

“Sorry, we don’t have Hungarian.”

I shook hands and nodded nicely to about 20,000 a day: the living image of the universal tribe of God. I told them about the visiting relics of a new Italian saint. (We brought a holy corpse to Krakow from Turin for the event. In a specially-painted van. How Catholic can you get?) Then I sent them out back to a temporary coffee tent in our usually-forbidden monastic garden. They loved it.

This was World Youth Day (more properly, “Days”) in Krakow: the global Catholic love-fest and mass encounter of young people with the pope that has been going since 1987, about once every three years. It’s like the Catholic Olympics. Instead of ski, swim or sprint, we make art, sing mightily, and pray, smeared with sunblock, on some big open field. Held in a new place each time, it lasts a week and ends with your knees in the grass at a giant mass. It is a particularly brilliant idea of John Paul II.

I myself have been a World Youth Day pilgrim (which is what we call ourselves). I was there in Denver in 1993, as a 16-year-old hauled across country in his parents’ station wagon. I was there in Rome in 2000, the year of the Great Jubilee, as a recent college grad and commentator for Vatican Radio.

Now all of that came back to me, and more besides. There was my working partner from New York in 2001; my roommate from Rome, who is now a priest of that city. A hardy nun who worked in the kitchen in Warsaw, at the community I lived in when I was learning Polish, showed up looking plumper — a déformation professionnelle. There was my college chaplain; the bishop of the town where I grew up…

All around, in church and out: music. Real music, by amateurs untouched by Auto-Tune — the songs I heard at 16 and at 23, and on top of them all the Polish songs I have learned since. It’s rare to hear music on the street and know it’s not coming from a box — just the sound of flesh and breath, plus inspiration.

It’s rare to see a festival with no trace of carnival — where instead of channelling Pan and hopping pub to pub, the singers go smiling and laughing from altar to altar, backpacks dangling and bare limbs flashing in slightly impious style, smiling like cherubs.

This holy revelry is powerful, when it works. And it feels deeply ingrained, very ancient — somehow. Why?

At the risk of seeming a thief, here’s what I find in my breviary: the psalm says, “When I remember these things, I pour out my soul in me: for I had gone with the multitude, I went with them to the house of God, with the voice of joy and praise, with a multitude that kept holy-day.”

This has all been going on for quite a while

At World Youth Day, whether in Rome or Poland or (as planned for 2019) Panama, a Catholic boy has the chance to go up to Zion for a great pilgrimage feast.