Everyone who sins also commits lawlessness. Sin is lawlessness.

 Although John is a mystic, he has a very practical mind; and, therefore, he will not leave righteousness vague and undefined. Someone might say, "Very well, I accept the fact that the only thing which proves that a man belongs to God is the righteousness of his life. But what is righteousness?" John's answer is clear and unequivocal. To be righteous is to love our brother men. That, says John, is a duty about which we should never be in any doubt. And he goes on to adduce various reasons why that commandment is so central and so binding. 