And he spake the saying openly ( παῤῥησία); literally, without reserve. This sudden announcement excited St. Peter. It was a new and startling communication. Peter took him, and began to rebuke him.
The word προσλαβόμενος indicates that he "took hold of him," to lead him apart, as though to have the opportunity of warning him with the greater familiarity and secrecy. So say St. Chrysostom and others.
Peter would not have his own confession of Christ thus evacuated, as it were; nor does he think it possible that the Son of God could be slain. So he takes him apart, lest he should seem to reprove him in the presence of the other disciples; and then he says (Matthew 16:22), "Mercy on thee, Lord ( ἵλεώς σοι κύριε): this shall never be unto thee."