Summa Theologica 51.3:
It was not fitting for Christ's body to putrefy, or in any way be reduced to dust, since the putrefaction of any body comes of that body's infirmity of nature, which can no longer hold the body together. But as was said above, Christ's death ought not to come from weakness of nature, lest it might not be believed to be voluntary: and therefore He willed to die, not from sickness, but from suffering inflicted on Him, to which He gave Himself up willingly. And therefore, lest His death might be ascribed to infirmity of nature, Christ did not wish His body to putrefy in any way or dissolve no matter how; but for the manifestation of His divine power He willed that His body should continue incorrupt. Hence Chrysostom says that with other men, especially with such as have wrought strenuously, their deeds shine forth in their lifetime; but as soon as they die, their deeds go with them. But it is quite the contrary with Christ: because previous to the cross all is sadness and weakness, but as soon as He is crucified, everything comes to light, in order that you may learn it was not an ordinary man that was crucified.Matthias Grünewald's biography is unknown; even his surname (Gothardt) has been misremembered. He was of the first generation of German artists to be influenced by Protestantism, but unlike Albrecht Dürer and Lucas Cranach, he left no record of his religious opinions. A chest of his belongings was posthumously found to contain Lutheran literature, but this in itself proves little. None of his surviving paintings was commissioned by Protestants; none contains any discernable Protestant message.
Since Christ was not subject to sin, neither was He prone to die or to return to dust. Yet of His own will He endured death for our salvation, for the reasons alleged above. But had His body putrefied or dissolved, this fact would have been detrimental to man's salvation, for it would not have seemed credible that the divine power was in Him. Hence it is on His behalf that it is written: What profit is there in my blood, whilst I go down to corruption? as if He were to say: If My body corrupt, the profit of the blood shed will be lost.
Christ's body was a subject of corruption according to the condition of its passible nature, but not as to the deserving cause of putrefaction, which is sin: but the divine power preserved Christ's body from putrefying, just as it raised it up from death.
What message they do contain is more elusive, and more disturbing.
Grünewald's most famous work, the Isenheim Altarpiece, has a very confused iconography; John the Baptist is the most prominent figure at the foot of the Cross and the Resurrection is bizarrely conflated with the Transfiguration. No art historian has ever identified beyond dispute the kneeling, glowing figure before a chorus of angels in one of the panels; she appears Marian, but worships a larger, older, homelier Mary in the facing panel. A widely accepted theory is that this is the idea of Mary in the mind of God adoring Mary as fact; one gets the sense that the art historians were desperate. Behind the mysterious woman, among the angels, a strange figure plays a viola de gamba; he is multiwinged and entirely covered in green feathers and curly hair. At least one scholar has argued that he is Lucifer.
That seraphic cellist on the inner panel of the Isenhem Altarpiece might represent the entire oeuvre of the painter; something is bowing discord; something disguised, perhaps something satanic. It is all the more frightening because it is impossible to prove anything about it.
Most troubling are the human figures. There is an indefinable wrongness to the bodies given by Grünewald to Christ and His saints; they are awkwardly posed, with lopsided heads, puffy skin, sausagelike fingers, almost cretinous expressions; they are seemingly moulded from glutinous bread dough. This is entirely unlike the precise grotesquery of Hieronymus Bosch (drawn from the tradition of comic manuscript marginalia) because it marks every face, not merely those of villains.
And then there are the Crucifixions. The corpus of the Isenheim Altarpiece, and the similar corpora in paintings at Karlsruhe, Basel and Washington are so well-known that detailed description is unnecessary.
Justifications for Grünewald's dead Christs invariably invoke the writings of St. Bridget, St. Gertrude and other late mediaeval mystics, who described the Passion in gruesome detail. And it is true that, beginning in the 14th century, intense meditation on the physical suffering of Christ dominated spiritual writing as well as sacred art. The pious literature of the age even numbers the lashes that Our Lord received (5,475 according to Oliver Maillard). Late mediaeval art is morbid, somber and tragic; its crucified Christ is crowned with thorns, contorting under His own weight, streaming blood. His muscles and bones are visible beneath His stripped-away skin. This is the suffering Christ bearing the immeasurable sins of humanity.
But this is most emphatically NOT the Christ whom we encounter at Isenheim and at Karlsruhe. The difference is obvious; 5,475 scourges turn a body red, not green - and Grünewald's Christ is green. It is the corruption of the tomb that turns a body green. Grünewald's Christ does not bleed; he rots.
This is not, as Joris-Karl Huysmans fatuously claimed, the Christ of Justin, Basil, Cyril, Tertullian, the Christ of the apostolic Church... the Christ of the afflicted, of the beggar, of all those on whose indigence and helplessness the greed of their brother battens; this is not their Christ at all. This is a Christ unknown to the mystics, unknown to the fathers, unknown to the poor and suffering; a Christ unknown to any Christian.
It is a Christ who has never risen from the dead.