1 John 3:7
 Little children, let no one deceive you. Whoever practices righteousness is righteous, as he is righteous. (ESV)
Maybe you have heard the expression, "If it acts like a duck, if it quacks like a duck, it's a duck." What's the point? The point is that actions tell the true story. There are many who claim to be something that they are not. In the Early Church false teachers were bringing deceptive words to sway the believers into their falsehood. They did not see fit to live a godly lifestyle yet they felt that they could claim to be righteous.
John says, "let no one deceive you." Anyone can claim to be righteous, but the one who is genuinely righteous is the one who acts righteously. If it ain't quacking, it's not a duck. John is speaking of new habits. New actions. Whoever practices righteousness is righteous. This stands in contrast to the practice of lawlessness that he has described in previous verses. Basically, those who do not know the Lord are habitual sinners. Those who do know the Lord take on new habits of righteousness.
This passage has everything to do with our nature. We either have an old (sinful) nature or we have a new (righteous) nature. Our behavior will make it very evident which nature we have. There is a wonderful passage in Ephesians where the Apostle Paul talks about the sinful nature and new nature:
 to put off your old self, which belongs to your former manner of life and is corrupt through deceitful desires,  and to be renewed in the spirit of your minds,  and to put on the new self, created after the likeness of God in true righteousness and holiness. (ESV)
What self have you put on? If you know the Lord, put on the new self, created in God's image. We will all know that you have put on the new self if your actions back it up.