1 John 3:4

Everyone who sins breaks the law; in fact, sin is lawlessness.

(v4, NIV)

Sin is equivalent to lawlessness. What does lawlessness mean? It's disobedience to God and rejection of his ways. This will lead to false teaching and living in immorality. This may be linked to these antichrists John has been talking about. These people who left were teaching false doctrine. They disobey God and have rejected his ways. They are sinners.

Now, some may try to convince themselves that they aren't sinning. They may say that this isn't sin. Maybe they break a few rules here and there, but they tell themselves that it's not sinning. However, this is lawlessness. They are disregarding rules that are put in place for our benefit. We are in subjection to authorities and people who are over us. Disregarding their rules is lawlessness. More importantly, disobeying and disregarding God is lawlessness. Of course, if a rule is set that is in direct opposition to God, then it's probably better to obey God than to obey men. Regardless, sin is lawlessness. Call it what it is and turn from it.