What does John 16:31 mean?

Much of Jesus' teaching could not be fully understood until after His resurrection (John 2:22) and with the help of the Holy Spirit (John 16:12–13). That didn't stop the disciples from frequently being overconfident in their own understanding. Peter was sharply corrected by Christ earlier in this same conversation, for that reason (John 13:36–38). When Jesus began to promise that He would begin speaking plainly and directly to His followers (John 16:25), the disciples seem to have assumed they had already been given the spiritual clarity Jesus predicted (John 16:29–30).

Christ's response is almost always translated in the form of a question. The upcoming context shows He knows the disciples fail to understand what is about to happen. This statement is challenging, and comes across as a gentle form of sarcasm: oh, you understand, do you? In truth, there is still a time of fear and doubt yet to come (John 16:20–22). Just as Peter had yet to go through his denial of Christ (John 13:36–38), the entire group of disciples will initially react to Jesus' arrest (John 18:1–3) in fear and panic (John 16:32).