A.T. Robertson Commentary Matthew 16:22

"And Peter took him, and began to rebuke him, saying, Be it far from thee, Lord: this shall never be unto thee." — Matthew 16:22 (ASV)

Peter took him (προσλαβομενος αυτον ο Πετρος). Middle voice, "taking to himself," aside and apart, "as if by a right of his own. He acted with greater familiarity after the token of acknowledgment had been given. Jesus, however, reduces him to his level" (Bengel). "Peter here appears in a new character; a minute ago speaking under inspiration from heaven, now under inspiration from the opposite quarter" (Bruce). Syriac Sinaitic for Mr 8:32 has it "as though pitying him." But this exclamation and remonstrance of Peter was soon interrupted by Jesus.

God have mercy on thee (ιλεως. Supply ειη or εστω ο θεος).

This shall never be (ου μη εστα σο τουτο). Strongest kind of negation, as if Peter would not let it happen. Peter had perfect assurance.