The Pharisees and Sadducees came and demanded that Jesus show them a sign from heaven (see Matthew 16:1). Jesus responded, “A wicked and adulterous generation seeketh after a sign; and there shall no sign be given unto it, but the sign of the prophet Jonas” (Matthew 16:4). Later, Jesus called His disciples together and asked, "Whom say ye that I am? And Simon Peter answered and said, Thou art the Christ, the Son of the living God" (Matthew 16:15-16).
Jesus declared, "Blessed art thou, Simon Barjona: for flesh and blood hath not revealed it unto thee, but my Father which is in heaven" (Matthew 16:17). Christ was saying, "You didn't get this revelation just by walking with Me, Peter. My Father revealed it to you from heaven." In short, Peter received the glorious, initial revelation that comes to everyone who believes. The glory of Christ's salvation was being revealed in him.
Yet, we read, "Then charged he his disciples that they should tell no man that he was Jesus the Christ" (16:20). Why did Jesus say this? Hadn't heaven itself already announced that He was the Lamb of God who had come to save the world?
The fact is, the disciples weren't ready to testify of Him as the Messiah. Their revelation of Him was incomplete. They knew nothing of the cross, the way of suffering, the depths of their Master's sacrifice. Yes, they had already healed the sick, cast out devils and witnessed to many. But even though they had been with Jesus for those years, they still had no deep, personal revelation of who He was.
The next verse confirms this: "From that time forth began Jesus to show unto his disciples" (16:21). In other words, Christ began to reveal Himself to them, showing them deeper things about Himself. The rest of the verse continues, "how that he must go unto Jerusalem, and suffer many things of the elders and chief priests and scribes, and be killed, and be raised again the third day."