1 John 3:9
Sometime in the past, one may have heard that "cannot sin" applies to Christians when resurrected as spirit beings. This is probably not correct because the whole context of the passage involves the here and now—today, during our physical lives. John is describing a situation in which we have opportunities to sin or not.
"Cannot sin" does not mean that it is impossible for us to sin, but rather, it is an act that we will not permit ourselves to do. Many of us have likely said to a child, "You can't do that!" Yes, they could do it, but we have determined that it is totally unadvisable. This is the gist of John's meaning: A person who is born of God is unable to sin habitually.
Why? Because of the divine nature being within him! This does not mean that he will not slip or that he will not even sin willingly and willfully from time to time, knowing full well what he is getting into. There is still weakness in human flesh. However, the converted person will repent and fight the weakness tooth and toenail. He will not live in sin! God will not abide in sin, and if His Spirit is within us, and we choose to continue in sin, then He will withdraw His Spirit.
John W. Ritenbaugh
The Covenants, Grace, and Law (Part 8)