According to the Dutch scholar and artist Peter Voorn, the famous Ghent Altarpiece - also known as the Mystic Lamb - painted by Flemish Primitives Jan and Hubert Van Eyck is filled with crypto-iconography. Instead of a portion Catholic mysticism, the panels of the Ghent Altarpiece give us an idea of what was going on at the Council of Constance (1414-1418). Peter Voorn's theory dates from 1999 and was partly published in the Jahrbuch der Oswald von Wolkenstein Gesellschaft (Germany). He works on a book on the subject, but here are some significant aspects of this new interpretation.

In the 15th century, St Bavo's Cathedral of Ghent (Belgium) was in fact a smaller parochial church named after its patron St John the Baptist. In the pentagonal Vyd Chapel of this church, the Altarpiece was placed on May 6, 1432, shortly after it was finished, on the feast-day of St John the Evangelist. It consisted of 24 or 25 panels. One panel was a "predella", supporting the altar, that probably was destroyed somewhere in the 15th century, because it showed a picture of hell. Paintings of the underworld were frequently damaged in those days.

The small chapel got its name from Joos Vyd, a wealthy citizen and even a major of Ghent for a while, who gave the order to built the chapel for the very huge painting. After the death of Hubert Van Eyck in 1426, Joos Vyd and his wife Elisabeth Borluut sponsored Hubert's brother Jan - also a painter, and a diplomat too - to finish the work. Some time later Vyd established a foundation to protect the Mystic Lamb in the years to come. This was confirmed by a quatrain found in the 19th century, confirming the names of painters and sponsors and the day it was finished.

And here the story starts for sure:
http://hubpages.com/hub/The-Van-Eyck-Cryptogram