Here lies the great distinction between the children of God and the children of the Devil.  The person who does not daily live for righteous ends, and in a righteous manner, is not of God, no matter what he may pretend.  He can be as religious as he pleases, but if his religion is not joined with practical sanctification, ten thousand ceremonies and almsdeeds will do him not an ounce of good.  His unregenerate heart is most clearly demonstrated by this: he does not love his brother.  He has no deep, abiding love for the children of God, and this betrays the fact that wickedness that has not yet been rooted out of his heart by regeneration.

Love to the brethren will be a primary emphasis throughout the rest of the epistle.  John wants us to be in no doubt that is one of the definite distinguishing marks of the genuine child of God.  He will clarify it in such crystal clear terms, that it becomes a virtue that becomes very difficult to successfully practice, even for one born again.  Each professing believer must strive to cultivate the kind of self-sacrificing love to our fellow Christians that John insists upon, and to examine ourselves for this necessary fruit.