Arnold Wion, a Belgian Benedictine, printed the prophecy in 1595 in his history of the Benedictine order, Lignum Vitae, crediting it to Malachy of Armagh on the authority of a manuscript he said he had seen. Malachy died in 1148 after a career unusually well documented for a twelfth century churchman: Bernard of Clairvaux, who knew him personally, wrote his biography within a few years of his death, and neither Bernard nor any other medieval source mentions a prophecy of the popes at all. The text is absent from every catalogue of Malachy's writing until Wion's book, a silence of four and a half centuries that is itself evidence.
The internal evidence points the same way. The first seventy four mottoes, covering popes from Celestine II in 1143 to Urban VII in 1590, fit their subjects closely enough that scholars have traced the wording to Onofrio Panvinio's printed history of the popes, published in 1557, four centuries after Malachy's death. After 1590 the fit falls away sharply: the mottoes turn vague enough to be stretched onto almost any pope who follows, exactly what a forger writing up to his own moment, then guessing, would produce.
Claude-Francois Menestrier, a French Jesuit, examined the claim in the 1680s and 1690s and rejected it, arguing the list was composed around the 1590 conclave, plausibly to promote a candidate whose name matched one of the mottoes. His account remains the starting point for the modern case against authenticity, restated at length by the nineteenth century historian M. J. O'Brien and accepted by most historians since: a late sixteenth century composition, not a medieval one, whose manuscript, if one specific copy ever existed, was almost certainly the forgery itself.
Sources Lignum VitaeArnold Wion with An Historical and Critical Account of the So-Called Prophecy of St. Malachy Regarding the Succession of PopesM. J. O'Brien