Ed Simon at Hyperallergic:
Though firmly a Renaissance painter in regard to technical acumen, Grünewald was Medieval in his vision, an inheritor of the 14th and 15th centuries’ comfort with the horrors of embodiment. In Germany, in particular, there existed then a form of devotion that took succor precisely in visualizing the sort of gruesome tableaux that Grünewald expertly painted, especially as regards the paradox of an almighty God himself suffering and dying. “Be assured of this,” wrote Suso’s contemporary, the mystic Thomas à Kempis in The Imitation of Christ (c. 1418–27), “that you must live a dying life.” The so-called Imitatio, whereby the penitent would imagine the degradations of the passion, was common in Medieval Catholicism, and reinvigorated by the Ignatian exercises of the Jesuits centuries later.
As such, nothing is philosophically novel in the Isenheim altarpiece, with its Christ who appears slick with the clammy sweat of death, stinking with the purification of the sepulcher. But to see something so ugly so perfectly depicted remains shocking five centuries later.
more here.