Zurbarán employs a standard iconography, and this painting is remarkably similar to others of the period, such as that by his patron Diego Velázquez.
This crucified Christ is serene. The image owes more to John's Gospel, where the Lord Jesus is absolutely in charge up to the moment of his death when he proclaims “It is finished,” than to the agonized and human Jesus of Mark's Gospel who cries out “My God! Why have you forsaken me?” Though evidently dead, Zurbarán's Jesus seems to stand upright, more supporting himself then hanging from the cross.
Still, Renaissance realism is the order of the day. This Christ has no halo.
The work is masterful, painterly and perhaps a bit show-offy. See how realistically the artist renders the human form. See how well he has mastered the play of light and dark. See how accurately he represents the textures of the wooden cross, the iron nails, the scrap of paper beneath Jesus' feet that bears the artist's signature, the excessively large and billowy loincloth. (Zurbarán was apparently noted for his rendering of white cloth!)
The impenetrably black background is a reminder of the darkness that, according to the Synoptic Gospels, “fell over the whole land” when Jesus hung on the cross. It also serves to focus the viewer's attention on the painting's sole subject.
I think that the artist has put all of his technique into the service of a message. Like the Apostle Paul, Zurbarán proclaims only “Christ, and him crucified.” (1 Corinthians 2:2)