Reconstruction of the church began in 1384, funded by a papal indulgence, and finished around 1400: a three-aisled cruciform hall church in north German brick Gothic, far too big for the place, which is how you can still tell what happened here. Pilgrims came in numbers that made Wilsnack one of the great destinations of fifteenth-century Christendom, spoken of alongside Santiago de Compostela. A recognised route ran from the Marienkirche in Berlin north-west across the plain. Others walked from much further. From 1396 pilgrim badges were being cast in Havelberg — small lead tokens in the shape of three hosts, bought at the shrine, worn on the hat, dropped in rivers and lost in ditches all over northern Europe. Archaeologists are still finding them.

The Prignitz had been poor. Then, quite suddenly, it had a revenue stream that arrived on foot and paid for lodging, food, candles and badges. In 1395 Johann III Wöplitz, Bishop of Havelberg, folded the Wilsnack church into his own episcopal household, an arrangement under which two-thirds of the income flowed directly to the bishopric. This is the detail that Luther and others would seize on more than a century later, and it is hard to argue with the logic: a church that profits from a wonder has an interest in the wonder continuing. Nobody at the time claimed the money was irrelevant. The dispute was over whether the blood was real.

Objections started early and came from serious people. Jan Hus attacked the cult in 1403, which mattered because Bohemians were among the pilgrims. In 1405 Zbyněk Zajíc of Hazmburk, Archbishop of Prague, appointed a commission to examine the shrine; it reported that the miracles were fraudulent, and the archbishop forbade anyone under his jurisdiction to make the journey. Later Nicholas of Cusa, as cardinal-legate, tried to stop the pilgrimage outright. The compromise came from Pope Eugene IV, and it was a small masterpiece of institutional face-saving: the shrine could stay open, but a freshly consecrated host had to be displayed beside the relics, so that whatever the pilgrims were venerating, at least one item on the altar was unambiguously the real thing. The argument ran on for more than a century.

The crowds did not always behave the way the church wanted. Around 1475 groups of children set off for Wilsnack on their own, a wave of youthful pilgrimages that drew in vagabonds and drifters and alarmed every town along the way; Erfurt dealt with it by locking its gates. Four decades earlier the English mystic Margery Kempe had reached Wilsnack in 1433 — one of the very few pilgrims to this shrine who left a written account of herself. Over roughly 170 years, hundreds of thousands of people made the journey — most of them leaving no record beyond a lost lead badge and the size of the church their money built.

The end came from inside. Wilsnack had turned Protestant, and its evangelical pastor, Joachim Ellefeld, regarded the hosts as an offence that his own congregation was still walking past every week. On 5 June 1552, against the explicit orders of the town council, he broke open the monstrance and burned the three wafers. The Havelberg cathedral chapter had him imprisoned at Plattenburg; Elector Joachim II then had him released and exiled. Nothing of the relics survives, and the pilgrim routes emptied. What remains is the church, still enormous, still known locally as the Wunderblutkirche, standing over a spa town now called Bad Wilsnack — a building whose scale is the only surviving evidence of what once happened inside it.