Matthew 16:21

From that time began (apo tote hrxato). It was a suitable time for the disclosure of the greatest secret of his death. It is now just a little over six months before the cross. They must know it now to be ready then. The great confession of Peter made this seem an appropriate time. He will repeat the warnings ( John 17:22 f. with mention of betrayal; John 20:17-19 with the cross) which he now "began." So the necessity (dei, must) of his suffering death at the hands of the Jerusalem ecclesiastics who have dogged his steps in Galilee is now plainly stated. Jesus added his resurrection "on the third day" (th trith hmerai), not "on the fourth day," please observe. Dimly the shocked disciples grasped something of what Jesus said.