From that time Jesus began to show his disciples that he must go to Jerusalem and suffer many things from the elders and chief priests and scribes, and be killed, and on the third day be raised. Matt. 16:21, RSV.
By now it had become evident that Israel as a corporate body was not going to accept Jesus as the Messiah. That left a major task for Jesus to accomplish: to prepare the disciples for His death.
Matthew 16:21 is the first explicit announcement of that fact. We should be aware of each of its elements. He
1. "must go to Jerusalem,"
2. "Suffer many things from the elders and chief priests,"
3. "be killed,"
4. "and on the third day be raised."
It wasn't as if He hadn't alluded to some of those events before. But now it was time for straight talk. Thus the significance of "Jesus began." He must teach those things openly and explicitly.
And why, we need to ask, did Jesus choose this precise time to set forth such important teachings? Because Peter's confession that He was the divine Christ indicated that he and the other disciples had begun to gain insight. They now knew who Jesus was. But it was one thing for them to confess that Jesus was the Messiah, but quite another for them to understand the nature of that Messiahship. The disciples had visions of glory and triumph in their heads, but Jesus knew that His end would be death and rejection. As He saw the plotting of the religious powers, He realized that it was vital to instruct His followers on the true reality of His mission.
Why the urgent necessity? Without the knowledge of His forthcoming death, it would have completely shattered their faith. And even with it their faith bordered on being wiped out. But He told them before it came to pass, so that when it did they might believe (John 13:19).
Earlier on, Jesus could not have given such information. Had He done so, because of the universal conception the Jews had of a kingly Messiah, the disciples would have rejected Him outright. They would have refused to believe because, as far as they were concerned, Jesus didn't even know what Messiahship was all about. But now, because they knew who He was, He could explain to them what His mission was.