Vincent's NT Word Studies
21. From that time began (apo tote hrxato). He had not shown it to them before.Must (dei). It was necessary in fulfilment of the divine purpose. See Matt. xxvi. 54; Heb. viii. 3; Luke xxiv. 26.
Suffer. This first announcement mentions this passion and death generally; the second (xvii. 22, 23), adds his betrayal into the hands of sinners; the third (xx. 17-19), at length expresses his stripes, cross, etc. Elders and chief priests and scribes. A circumstantial way of designating the Sanhedrim, or supreme council of the Jewish nation.
Robertson's NT Word Studies
16:21 {From that time began} (apo tote erxato). 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 qen. The great confession of Peter made this seem an appropriate time. He will repeat the warnings (#17:22f. with mention of betrayal; #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" (tei tritei hemerai), not "on the fourth day," please observe. Dimly the shocked disciples grasped something of what Jesus said.