I believe it’s much more likely that the Letter of Approbation was genuine, but that the Malleus itself was never actually read by the gentlemen who endorsed it. I think it’s much more likely that Dr. Edward Peters was correct when, in his section of the work Witchcraft and Magic in Europe: Volume Three – The Middle Ages [page 239], he stated; “The approval of the theological faculty of Cologne was arranged through a complicated series of academic negotiations – it, too, does not address the remarkable qualities of the work itself. It is doubtful whether Innocent VIII or the theological faculty of Cologne ever read the work.”
Also, according to Dr. Christopher Mackay, whose recent translation represents a reliable modern scholarly edition of the Malleus Maleficarum, “The argument was made in the nineteenth century by a scholar hostile to what the Malleus stood for that the approbation was a forgery by Institoris and that Sprenger had nothing to do with the composition. The evidence for this is in my view very tenuous (and the main argument is clearly invalid). Nonetheless, once the argument was put forward, it took on a life of its own, and people continue to advance arguments in favor of the idea that Sprenger’s involvement was a falsification perpetrated by Institoris, despite the fact that this argument was vitiated from the start.”
Whether or not the work was ever officially banned by the Catholic Church, the Malleus Maleficarum became the de-facto handbook for witch-hunters and Inquisitors throughout Late Medieval Europe. Between the years 1487 and 1520, it was published thirteen times, and between 1574 to 1669 it was again published sixteen times.
The Malleus Maleficarum perhaps owes most of its popularity to Johannes Gutenberg. It was the invention of the printing press in the middle of the fifteenth century that allowed the work to spread so rapidly throughout Europe.