These words are a quote from Isaiah 53:9 (using the Septuagint, the Greek translation of the Old Testament). The purpose of this quote is not to prove a point in Peter’s argument so much as to draw out, using Isaiah’s words, the deep comfort there is for oppressed slaves in Jesus’ unjust suffering. By quoting this verse, Peter implies that the entire “Servant Song” of Isaiah 52:13 – 53:12 is about Jesus Christ; that is, it describes his suffering complete with the reason why he suffered: “for us.” If Jesus did not resist the abundance of unjust abuse that came his way for our benefit, the slaves that Peter addresses ought readily to follow in his footsteps and accept the injustice that comes their way—thus drawing attention to Christ.
22 He committed no sin, neither was deceit found in his mouth.