Did Peter lose his salvation when he denied Jesus?
No, Peter did not lose his salvation when he denied Jesus. His bitter weeping showed conviction of sin, not loss of salvation. He was reconciled immediately upon confessing his sin, just as a believer in Paul's doctrines of grace is restored.
Les explains that Peter's denial was real sin, but it did not throw him out of salvation. Jesus had prayed for Peter that his faith would not fail when Satan sifted him like wheat. After the cock crowed, Peter wept bitterly because he was convicted of his sin. The context asks, "Did it throw Peter out? No." He was reconciled immediately when he confessed his sin, and so it is with a believer in Paul's doctrines of grace. Paul never gives license to sin, and John writes that believers should not sin, but if they do, there is provision.