http://www.christian-faith.com
The apostle John gives a very clear and straightforward way of discerning between the children of God and the children of the devil.

The children of God all practice righteousness, and love the brethren.
Those who don't, who practice sin, are of the devil, even if they are church members.

It seems that John is encouraging in his hearers the capacity to discern and know the difference.

Jesus said that A tree is known by its fruit. He said, "Beware of false prophets. By their fruit you shall know them."

It all fits in together. If your life doesn't fit this doctrine, change your life!