I’ve made my share of artistic pilgrimages. But last year I set out on the ultimate quest: I took the Eurostar to Belgium. The crowds who squeeze into a tiny chapel in St Bavo’s Cathedral to see Hubert and Jan van Eyck’s Ghent Altarpiece are not there merely because there’s not much else to do in Ghent. As we wait for the doors of this giant painting made of multiple hinged wooden panels to open, I run through the reasons it’s supposed to be so special.

There are so many firsts in this painting you can lose count. It contains the first realistic interior, the first convincing townscape, the first great north European landscape, the first completely lifelike portraits. Begun by Hubert in the 1420s, it was finished in 1432 by his brother Jan. That’s at the very beginning of the Renaissance, long before such heights of realism reached Italy.

Yet some works of art that were revolutionary in their day become senescent with time. Not this one. One of the strangest Annunciation scenes ever painted is spread across the closed panels. The angel visits Mary in a room whose lovely details include a washbasin in a Gothic niche. Out of the window we see a medieval town painted with an acute eye for architecture and space.

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This is intimate, it’s so human – then the doors open to reveal a sublime vista. Christ sits enthroned in heaven, accompanied by Mary in a blue gown and gold crown. Why did the Van Eycks reject the conventional focus in European art on the suffering of a human Christ on the cross and instead reach back to something more primitive and barbaric, an omnipotent eternal judge? And another first: Adam and Eve stand naked, their embarrassed nudity not only unprecedented in its day but still uneasily real and touching to look at.

The divine and human, real and fantastic – these almost shocking contrasts display the artists’ sophistication. The lower part is a visionary landscape that spreads across, and connects, five separate panels. Exotic nature and unearthly cities unfold under a glowing sky. This is not just the start of the Renaissance but Romanticism too.

There is so much to look at that you barely notice one of the panels is a modern copy. If the lost original of the Just Judges really does come to light it will add yet more magic to a masterpiece that starts by involving you in humble depictions of the real then transports you to mysterious realms of revelation. The Ghent Altarpiece may be just a train journey away but it is a passport to paradise.