A.T. Robertson Commentary Matthew 16:20

"Then charged he the disciples that they should tell no man that he was the Christ." — Matthew 16:20 (ASV)

That they should tell no man (ινα μηδεν ειπωσιν). Why? For the very reason that he had himself avoided this claim in public. He was the Messiah (ο Χριστος), but the people would inevitably take it in a political sense. Jesus was plainly profoundly moved by Peter's great confession on behalf of the disciples. He was grateful and confident of the final outcome. But he foresaw peril to all. Peter had confessed him as the Messiah and on this rock of faith thus confessed he would build his church or kingdom. They will all have and use the keys to this greatest of all buildings, but for the present they must be silent.