Question: "What was the reason behind Peter's denial of Jesus?"
Answer: The story of Peter’s threefold denial of Christ is found in all four Gospel accounts: Matthew 26:69–74, Mark 14:66–72, Luke 22:55–62, and John 18:15–18, 25–27. But why would the chief of the disciples deny even knowing Him? There were two main reasons why Peter denied Jesus: weakness and fear.
Peter’s denial was based partially on weakness, the weakness born of human frailty. After the Last Supper, Jesus took His disciples to the Garden of Gethsemane to await His arrest. He told them to stay awake and pray while He went off to pray alone. When He returned to them, He found them sleeping. He warned Peter to stay awake and pray because, although his spirit might be willing, his flesh was weak. But he fell asleep again, and, by the time the soldiers had come to arrest Jesus, it was too late to pray for the strength to endure the ordeal to come. No doubt his failure to appropriate the only means to shore up his own weakness—prayer—occurred to him as he was weeping bitterly after his denials. But Peter learned his lesson about being watchful, and he exhorts us in 1 Peter 5:8, “Be on the alert, because your adversary, the devil, prowls around like a roaring lion, seeking someone to devour.” Peter’s weakness had caused him to be “devoured” momentarily as he denied his Lord because he hadn’t been prepared through prayer and he underestimated his own weakness.
A second reason for Peter’s failure was fear. To his credit, although all the others had fled (Mark 14:50), Peter still followed Jesus after His arrest, but he kept his distance so as not to be identified with Him (Mark 14:54). There’s no question that fear gripped him. From the courtyard, he watched Jesus being falsely accused, beaten, and insulted (Mark 14:57–66). Peter was afraid Jesus would die, and he was fearful for his own life as well. The world hated Jesus, and Peter found that he was not prepared to face the ridicule and persecution that Jesus was suffering. Earlier, Jesus had warned His disciples as well as us today, “If the world hates you, keep in mind that it hated me first” (John 15:18; cf. Matthew 24:9). Peter quickly found he wasn’t nearly as bold and courageous as he had proclaimed, and in fear he denied the One who had loved him.
We might well wonder why Jesus allowed Peter to fail so miserably and deny his Lord three times that night. Jesus revealed to Peter that Satan had asked for permission to sift Peter like wheat (Luke 22:31). Jesus could have easily protected Peter and not allowed Satan to sift him, but Jesus had a higher goal. He was equipping Peter to strengthen his brothers (Luke 22:32). Not only did Peter strengthen the other disciples, but he became the pillar of the early church in Jerusalem, exhorting and training others to follow the Lord Jesus (Acts 2). And he continues to this day to strengthen us through his epistles, 1 and 2 Peter. As with all our failures, God used Peter’s many failures, including his three denials of Christ, to turn him from Simon, a common man with a common name, into Peter, the Rock.