What does John 16:30 mean?

Throughout Jesus' earthly ministry, He has taught using parables and stories (Matthew 13:10–13; Mark 4:33–34). One reason for that tactic is that those stories are more likely to be remembered, and passed along, than dry explanations of theology. They are also ways for His followers to retain teachings which they could not fully understand at that moment (John 2:22). Using statements that require explanation also helps to weed out those who aren't really interested in truth anyway (Matthew 13:13).

Much of what Jesus has said during this Last Supper (John 13:1–5) has been blunt. He has recently promised that He will communicate with the disciples in a more direct fashion (John 16:25–28). Their immediate reaction is to assume that the spiritual clarity promised by Jesus has already arrived (John 16:29). Here, they imply that what they now see and hear from Jesus fulfills that knowledge.

The disciples, in a sense, are telling Jesus they now understand all that He has been saying to them. His response, starting in the next verse, is like His reaction to Peter's overconfidence (John 13:36–38). Christ will ask rhetorically—almost sarcastically—if it's true that they now fully "believe" and understand these things (John 16:31). What these men have already forgotten is that they still have spiritual "birth pains" to experience before all is made clear (John 16:20–22, 32).