What does John 13:7 mean?

Many lessons taught to the disciples are introduced long before they are understood. In some cases, these men need to see how Jesus' role as Messiah will be fulfilled in order to put together all the pieces (John 2:19–22). In other cases, like this, they need a drastic dose of humility. Jesus has surprised them by performing a servant's task: washing the disciples' feet. Later, He will explain the example this should set (John 13:12–20).

For now, the disciples are too concerned with power (Luke 9:54), prestige (Mark 9:34), and the thrill of recent events (John 12:12–13). Only after they see the extent to which Jesus humbles Himself (Philippians 2:8) will they grasp the significance of this foot washing. Peter, especially, will mature from this night's arrogance (John 13:37) into someone deeply aware of the need for humility (1 Peter 5:5).

"You don't understand now, but you will" is a clear response to Peter's objection. But Peter has always been too hard-headed to listen the first time around (Mark 8:31–33). He will reply with a flat-out, absolute rejection of what Jesus is attempting to do (John 13:8), then completely reverse direction and once again fall short of full understanding (John 13:9).