In the bible, during the Last Supper of Christ’s life he reveals that one of his disciples will betray him, which accounts for the animated discussion in this scene. We know the traitor to be Judas Iscariot, who later kissed Jesus and revealed his identity to the Roman soldiers who had come to arrest him. It was hard to track down information on this painting, but I believe that the figure of Judas here is the man on the left in the dark green robe, gloomily sipping wine, already resigned to his terrible fate.
Jesus also reveals at this supper that Peter will deny knowing him three times before the rooster crows the next morning. The disciple Peter sits on Jesus’ right side (the viewer’s left) and holds a knife, foreshadowing that he will cut off the ear of Malchus, the servant of the high priest of Israel. The youngest disciple, John, sits in front Jesus looking forlorn and quite frankly, a little bored.