1 John 3:4
 Everyone who makes a practice of sinning also practices lawlessness; sin is lawlessness. (ESV)
False teachers had infiltrated the church. Those who were trying to follow Christ were up against those who taught that material world was evil and only the spirit was redeemable or good. The by product of their false teaching led them to believe that they could do whatever they wanted with their physical body. The only thing that mattered was the spirit.
John's letter intends to correct this false teaching He responds to the sinful behavior that the false teachers are condoning by letting them know that sinning is lawlessness. We, who love the grace of God, also need to be careful. If we allow the doctrine of God's grace to excuse a lifestyle of sinful behavior we are practicing lawlessness and in grave error.
Jesus Christ came to fulfill the Law not abolish it. While it is true that the Law of God does not save us, it is also true that the God whom we serve is not a "lawless" God. As we become more and more like the God to whom we belong our life should look more like his. And that life in God is not a lawless life.
How do you view sin? Are you indifferent? Are you in the habit of sinning while excusing your behavior? If so, your lawlessness brings into question whether you truly know the Savior.