Sin and the Child of God
In discussing "purity", the subject of "sin", naturally arises, which may be why John moves right into a discussion of "Sin and the Child of God" (1 John 3:4-9). It also fits into John's overall theme of combating the Gnostic-like influences that were teaching that sinning did not affect one's relationship with God.
How shall the true child of God regard sin? Is it something to be taking lightly? Not if the apostle John has anything to say about it!