What does John 18:27 mean?

Twice, Peter has been asked about his relationship to Jesus, in relatively gentle ways (John 18:17, 25). The servants of the high priest who asked those questions assumed Peter was not a disciple of Jesus. Or, at least, they couldn't believe that any of Jesus' disciples would be stupid enough to be in the high priests' courtyard at that moment. In both cases, Peter has denied that he knows Jesus.

The third question is more direct. The servant who is asking the question was there when Jesus was arrested, so he would have seen Peter slashing with a sword (John 18:10–11). Worse, for Peter, the man challenging him now is related to the maimed servant. So, he's quite sure that Peter is the man he's thinking of (John 18:26).

John's depiction of Peter's third denial is bland; other Gospel writers explain that Peter (Mark 14:71; Matthew 26:74) became desperate and insisted, through a curse on himself, that he was not a follower of Jesus. At that moment, Jesus' earlier prophecy is completed, as the rooster crows (John 13:38).

Luke includes an especially heartbreaking detail. This marks what is almost certainly the lowest moment of Peter's life. At the instant Peter makes his third denial of Christ, Jesus is being brought back outside for the next phase of His sham trial (John 18:24, 28). Just as Peter is angrily insisting he does not know Christ, the two lock eyes (Luke 22:61). Memory of Jesus' recent prediction floods into Peter's mind (Luke 22:34, 62) and he immediately breaks down, running away and sobbing (Matthew 26:75; Mark 14:72).