Peter answers Jesus' question with a question of his own. Many had called Jesus' words "an hard saying" and left (John 6:60, 66), and Jesus left the choice to the twelve. Barnes notes that "it was submitted to their choice." Peter's question looks at the alternatives and finds none. Barnes says it "implied their firm conviction that Jesus was the Messiah, and that he alone was able to save them." Matthew Henry puts Peter's thought this way: "It were folly to go from thee, unless we knew where to better ourselves." Peter spoke for all twelve; Gill observes that he "had now no suspicion of Judas."

Peter answers his own question at once: "thou hast the words of eternal life." He goes on, "we believe and are sure that thou art that Christ, the Son of the living God" (John 6:69). Jesus does not praise the confession. He replies with a sobering question of His own: "Have not I chosen you twelve, and one of you is a devil?" (John 6:70). John explains that He meant Judas Iscariot, "being one of the twelve" (John 6:71).

Peter's phrase "the words of eternal life" picks up what Jesus had just said: "the words that I speak unto you, they are spirit, and they are life" (John 6:63). Barnes explains Peter's meaning as "thou teachest the doctrines which lead to eternal life."

Barnes also notes why Peter saw no one else to turn to. The religious teachers of the day, the Pharisees, the Sadducees, and the scribes, were in his words "corrupt, and unable to guide them aright." Gill notes that "Lord" could be a title of respect, like "Sir," or an acknowledgment of Christ's authority and a claim of interest in Him, which carries in it a reason to stay with Him.

The KJV and ESV word Peter's confession in John 6:69 differently: the KJV reads "that Christ, the Son of the living God," the ESV "the Holy One of God."

This question comes at a turning point in John's Gospel. "Many of his disciples" turned away when His words became hard (John 6:60, 66). The twelve stayed, not because they understood everything, but because they had found no one else with words of life. Barnes notes that though Jesus' doctrines were mysterious, "they were the only doctrines that could instruct and save them." Matthew Henry's concise notes put the contrast simply: the disciples "fastened on that plain saying" of eternal life "and resolved to cleave to him, when others fastened on hard sayings, and forsook him."

Jesus had just said that no man can come to Him "except it were given unto him of my Father" (John 6:65). Matthew Henry reads that verse to mean that "faith is the gift and work of God," and Peter's confession shows that gift at work. Yet Jesus' reply about Judas (John 6:70) keeps the moment honest: a true confession spoken by one does not prove the heart of every person standing near him.