The Da Vinci Code claim that the figure to Jesus’s right in Leonardo’s Last Supper is Mary Magdalena traces back to a single source: a 1956 hoax by Pierre Plantard that Plantard himself confessed to under oath in a 1993 French court, ten years before Dan Brown wrote his novel. The actual answer is older and weirder. The Beloved Disciple John has been depicted as androgynous in Christian art since the 4th century. The 14th-century German Christus-Johannes-Gruppe sculpture type, made for Bodensee Dominican women’s convents, shows John reclining on Christ’s chest with eyes closed; the nun standing in front of the sculpture identified with John, who rested where she wished to rest. The convention belongs to a thousand-year tradition of bridal mysticism in which the soul is the bride of Christ, expressed in the explicitly erotic vocabulary of the Song of Songs (Bernard of Clairvaux, Mechthild of Magdeburg, Hadewijch, Teresa of Ávila, John of the Cross). Leonardo inherited the convention and amplified it through his own Florentine Neoplatonic interest in androgyny. The fusion of Leonardo plus Saint John plus the alchemical Hermetic androgyne happens later, in 1850s and 1890s Paris occult ritual. This article walks through seven centuries of painting the same feminine John, includes the comparison gallery of pre-Leonardo Last Suppers that show the convention was iconographic and not personal to him, and asks the underlying psychological question: why is the language of religious union always also the language of erotic union?