If we say that we have no sin, we deceive ourselves, and the truth is not in us.
Some have pointed out that the need for John's teaching here resulted from the most audacious immorality advocated, indulged, and rationalized by heretics such as Valentinus.  Irenaeus has a description of such views, which although later associated with the heretic whose name was given to the error, nevertheless existed early in the first century.