John says that a genuinely born again believer cannot sin, in other words, cannot continue an ungodly life of sin. John Calvin taught that if a person professed Christianity and continued to live an ungodly life then that might be evidence that they were not genuinely converted to Christ. Now this does not apply to a genuine believer struggling with sinful habit patterns that he or she is desperate to get the victory over. What we are talking about is the person who says "Jesus has forgiven all my sin, so I can just continue to live as I please and not have to change my ways and live a holy life. What's the point? I'm forgiven and released from punishment anyway." This is the sign of a false conversion, because repentance is an integral component of true conversion. "Repent and be baptised, and you shall receive the gift of the Holy Ghost" said Peter. Genuine believers love Jesus, love the Word, have a strong desire to please God and live a holy life before Him. They mourn deeply that they still have sinful habit patterns that still need to be dealt with, and are seeking God in prayer, sometimes weeping before Him, that He will remove these habit patterns from them.