I have always LOVED the Isenheim altarpiece because it shows us something really wonderful (and true) that the artist understood about Christ. Jesus' skin is all marked up in this painting for a very special reason. The monastery in Isenheim was run by the Brothers of St. Anthony, and served as a hospital. The Brothers of St. Anthony were especially skilled at taking care of people with some really painful skin diseases. The people who came to the hospital sometimes looked a lot like Jesus did here - big sores all over their bodies.
So, imagine being a patient at this monastery and walking into the chapel to pray. You feel miserable and you look up and see this picture of Jesus. Not only do you see a reminder of how he suffered to save you, but you notice his skin looks like yours. You see that Jesus is someone who knows what it's like to be you! He knows what your pain feels like, and like the Bible says he is always "interceding" for us when we don't even know how to describe what we feel...After showing one of her classmates, "The first thing she said when she saw it was, "So much pain!" Yes. Exactly. I replied, "That's why I love it. It shows Jesus as someone who knows our pain."
This reminds me of another facet of the Crucifixion story - Christ Body in the Tomb.
Only instead of pain it's "So Much Death!"
I first came across Christ Body in the Tomb after reading about it in The Idiot by Dostoevsky; whom it had a significant impact on.
And if you are wondering why Christs body looks so similar in both works (particularly the hands), they were done by contemporary Germany painters in the first half of the 16th century. The altarpiece is by Matthias Grünewald and the tomb by Hans Holbein the Younger.