The first thing that always strikes me about this painting is the size. It's nearly 10 feet tall, making it very close to life-sized. The second thing is just how realistic the figure is. Zurbarán's Jesus is idealized to be sure, but it's also a deeply humanized one. The face is individualized and the strong lighting that comes from somewhere outside the painting calls attention to anatomical details, like the musculature in his torso and the way his toes curl slightly over the too small platform.
When the painting was first shown in the monastery in Seville that commissioned it, people were awed. It was only visible from afar through a grill, and spectators were amazed by how three dimensional it seemed. Later commentators noted that it appeared to be a sculpture rather than a painting. This appearance is heightened by the fact that the scene doesn't appear within a historical context, but on a stark black background, strongly contrasting with Jesus' white figure. Painted at a time when Catholics were aggressively campaigning for new believers, this painting achieved its goal of evoking intense religious feeling.
Image Credit: Francisco de Zurbarán. The Crucifixion, 1627.