Again, John is going to give us a list (the same list) of the qualities that the children of God possess. And the first quality on this list (as it was for those who walk in the light) is that they renounce sin.
In the previous list, John presented three false stances regarding sin: 1. Salvation with unchanged lives, 2. Salvation through merited grace, and 3. Denial of guilt. Here John is more direct in his admonition to renounce sin. First, he defines sin. It is lawlessness—generally speaking, not in the sense of obeying certain laws but rather general rebellion against God. Then, he reminds us that Christ came and lived a life without that sin—without rebellion—and died to remove our sin and guilt. Knowing this, how could we continue to practice sin?
Instead, what we practice as children of God is righteousness.
Not that we are perfectly righteous and never struggle with sin ever again. The key ideas to grasp in this passage are what John means when he talks about “practice” and “inability” to sin.
When John talks about children of God not sinning, saying that they “cannot sin,” we would perhaps use the phrase “such things are not done.” In God’s family, sin is not something we do. It is contrary to our values. That does not mean members don’t make mistakes; but we certainly see mistakes as such. We renounce such behaviors and seek to avoid them. And that is where practice comes into play. Children of God practice righteousness. The seek to improve their behavior with God’s help and discipline. What the certainly do not do is practice sin. The goal is to develop righteous habits and to avoid or overcome sinful ones.