He may have been talking to His disciples directly—and not to the crowds—when He said this. It was not until after Jesus died, was buried, and was resurrected that His disciples first believed—really believed! Later on, Peter and John ran pell-mell to the grave where Jesus had been interred. Peter goes in. He looks and sees everything there—except Jesus. John peers in the doorway, and the gospel says, "And then this disciple believed." John was the first to believe. It took seeing the grave clothes in the sepulchre and no body of Jesus Christ in sight for him to finally get it. That act fulfills this little prophecy of Jesus': "Then you will know."
What momentous events they had just experienced—and they did not understand and believe. They did not really believe when He came into Jerusalem and was lauded by all the people, exactly fulfilling the prophecy of Zechariah (Matthew 21:1-5; Zechariah 9:9). They did not see it when He gave the Last Supper, changing the symbols. They did not recognize it when one of their own betrayed Him with a kiss. He even gave the sop directly to Judas after John had asked Him, "Which one of these is it going to be?" He saw it go from Jesus' hand to Judas' hand or into his mouth. The fulfillment was just a few minutes away from His utterance, and they still did not believe! They still failed to see how it was all coming together.
Of course, there was the crucifixion. How many prophecies were fulfilled in the crucifixion, in His burial? And they still did not believe! The three days went by and still no belief. They had God-in-the-flesh leading them through all these prophecies, and they still did not get it—until that point when comprehension dawned on John.
It is arrogant and puffed up of us to think that we have prophecy figured out. In many cases, we do not have the mind even of the disciples. We do not have the teaching from the very mouth of God as the disciples had. Of course, they were not converted at the time. But if they could not get it, seeing these things happening right before their eyes, will we be able to see prophecy working out in our time any more clearly? Are we so much more advanced?
We can know the possibilities, but we cannot be certain of the exact progress and timing of prophetic events. Until the prophecy is fulfilled, we should not be dogmatic. We must always approach these things with humility. Because we are clay in the Potter's hand, He gives us what we need to know. And, in many cases, what we think we need to know is not really "need to know" until after it has already happened. God has His own ways, and He is working out His purpose.