What does John 16:21 mean?
This analogy is part of Jesus' reassurance to His closest disciples. This statement is being made near the end of His teaching at the Last Supper (John 13:1–5). Soon Jesus will be arrested (John 18:1–3) and crucified (John 19:18). Though He will be resurrected (John 20:19), the days in between will be filled with fear and anguish for these men. In addition, Jesus has warned them—and, by extension, all future believers—about the persecution to be expected from the unbelieving world (John 16:1–3). His reason for the warning is so they don't respond to those events in panic or surprise (John 16:4).Most recently, Jesus has referred to the deep sorrow these men will experience, accompanied by the sadistic joy of the unbelieving world (John 15:21; 16:20). The uplifting aspect of this message is that their intense pain will soon turn into even-more-intense celebration. The analogy Christ uses here is that of a woman in childbirth.
Of course, Jesus is not suggesting a woman who has given birth literally does not remember it happened. In fact, it's a cliché for mothers to remind children of the pain of labor, when the child is being unruly. The point in this passage is those negative experiences are immediately outweighed by the joy of the newborn baby. The birthing experience is intense, agonizing, and even frightening. Yet, in comparison to the love of a child, birth pain is relatively short-lived and more easily set aside.
The disciples will soon experience a similar pattern. There will be severe agony, terror, and uncertainty as the unbelieving world murders Christ and scatters His followers (Matthew 26:31; John 16:32). Yet, those three days of misery (John 2:19) will be followed by a faith so joyous and powerful (John 16:22) it will literally change the world (Acts 1:1–9).