Can you imagine an ordinary person, not the Son of Man, who would not sin? Great saints spoke of themselves as great sinners. Yet here the apostle John speaks of "sinlessness" as something quite familiar: "We know..."
Perhaps the solution to this paradox, as in many situations of spiritual life, lies in the four-word formula: "already yes, not yet." Birth from God, like salvation and like entering the Kingdom, is a dynamic concept. It has not simply happened; it is happening in us, and as it happens we move away from sin, because we both guard ourselves from evil and leave the evil one nothing in us to "get hold of."