Mark 8:32

And he spake that saying openly
Concerning his sufferings, death, and resurrection from the dead. He not only spoke it before them all, but in plain words, without a figure; so that it might be, and was clearly understood by them; and he spake it as the word will also bear, not only very freely, but likewise boldly, with an undaunted courage, with intrepidity of mind; being not in the least discouraged, nor showing any concern or fear about what was to befall him:

and Peter took him, and began to rebuke him.
Peter might more especially be concerned at this free and open account Christ gave of his sufferings and death, because he had just now acquainted him, that he should have the keys of the kingdom of heaven; by which he might understand some high post in the temporal kingdom of the Messiah he expected; and immediately to hear of his sufferings and death, damped his spirits, and destroyed his hopes, and threw him into such difficulties he was not able to remove; and therefore he takes Christ aside, and very warmly expostulates with him about what he had said, and chides him for it, and entreats him that he would not think, or talk of such like things: the words of Peter are recorded by Matthew, (See Gill on Matthew 16:22).