Matthew 16:21

We have here Christ's discourse with his disciples concerning his own sufferings; in which observe, I. Christ's foretelling of his sufferings. Now he began to do it, and from this time he frequently spake of them. Some hints he had already given of his sufferings, as when he said, Destroy this temple: when he spake of the Son of man being lifted up, and of eating his flesh, and drinking his blood: but now he began to show it, to speak plainly and expressly of it. Hitherto he had not touched upon this, because the disciples were weak, and could not well bear the notice of a thing so very strange, and so very melancholy; but now that they were more ripe in knowledge, and strong in faith, he began to tell them this. Note, Christ reveals his mind to his people gradually, and lets in light as they can bear it, and are fit to receive it. From that time, when they had made that full confession of Christ, that he was the Son of God, then he began to show them this. When he found them knowing in one truth, he taught them another; for to him that has, shall be given. Let them first be established in the principles of the doctrine of Christ, and then go on to perfection, Heb 6:1.

— Matthew Henry's Concise Commentary (public domain)

From that time forth began Jesus to show unto his disciples--that is, with an explicitness and frequency He had never observed before. how that he must go unto Jerusalem and suffer many things--"and be rejected," (Mar 8:31; Luk 9:22). of the elders and chief priests and scribes--not as before, merely by not receiving Him, but by formal deeds. and be killed, and be raised again the third day--Mark (Mar 8:32) adds, that "He spake that saying openly"--"explicitly," or "without disguise."

— Jamieson-Fausset-Brown Commentary (public domain)