In Matthew 16:21, Jesus again predicts his death, specifically now how it must (Greek: dei) happen, in Jerusalem, as part of the plan of salvation, but the disciples still did not understand his path, for they had not understood about how the Bridegroom would be taken away from them (Matthew 9:15), nor about how the Son of Man would spend three days and nights in the heart of the earth (Matthew 12:40).
Here in Matthew 16:21, Jesus predicts his own death for the first time in very clear terms. Previously there had only been hints. His suffering did not come as a surprise to him. He knew that he would die, where he would die, and in what manner it would take place. He now reveals to them that all of this must (Greek: dei) happen as part of the divine plan of salvation for the redemption of humanity. At this stage the disciples still had much more to learn concerning the path of the Messiah. They had, after all, not understood Jesus’ words about the Bridegroom who would be taken away from them (Matthew 9:15), nor his words concerning the Son of Man who would spend three days and nights in the heart of the earth (Matthew 12:40).1
21 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.