The Chapel of St. John the Baptist was built on the initiative of Gonzalo González de Illescas, Chancery Court Magistrate and Advisor to the Catholic Monarchs and his wife, Marina de Estrada, as may be observed on the inscription on the chapel walls under the ribs of the vaults. The inscription tells us that work on the chapel was completed in the year 1492. This same inscription also specifies the exact date the altarpiece was installed - April, in the year 1504 - which is why we can date it so precisely - quite an exceptional feat where Flemish altarpieces are concerned!
This was probably when a plinth with painted wings depicting the busts of both founders of the chapel at prayer and their closest relatives together with the figures of St. Augustine (San Agustín) and St. Jerome (San Jerónimo) was added. In the side compartments of the predella itself, two figures hold the family coat of arms that is repeated on the dome of the vault and on the open tombs in the crypt.