This is Robert Hughes on the Isenheim Altar in Colmar, as quoted by Clive James in his article "Golden Boy," a review, in the New York Review of Books of January 11, 2007, of Hughes's Things I Didn't Know:
I had never seen such a frightening picture before. Of course, as in Bosch, its repulsive powers were wound into the very fabric of the skill with which it was executed. But could one imagine the Isenheim Altarpiece hanging in a church or an infirmary back in Australia, where art never spoke of real pain, let alone grotesque sickness or deformity? Would there be any public place for it in America, or anywhere else in the modern world? It was only then, gazing on Grünewald's enormous and deliberately wrought masterpiece, that I realized how deep the roots of euphemism and evasion were sunk in modern life; how alien, as a result, the entire "Expressionist" tradition in modern art had been to me having grown up in Australia—and would also have been had I grown up in North America.... Almost all I knew of past art, and that imperfectly, was its Apollonian tradition, which spoke of order, idealism, satisfied Eros. Somewhere beyond and below that stretched another continent of esthetic experience which had somehow to be discovered, and it was probably true that my life had been too happy and healthy for me to really grasp it. This, too, was part of the reason I had had to leave Australia and come to Europe.
Grünewald's altarpiece is, indeed, one of the greatest works of art I have ever seen. For another take on it, see the first part of W. G. Sebald's Nach der Natur, translated by Michael Hamburger as After Nature.