Adam Clarke

He that saith he is in the light - He that professes to be a convert to Christianity, even in the lowest degree; and hateth his brother - not only does not love him, but wills and does him evil, as the Jews did the Gentiles; is in darkness - has received no saving knowledge of the truth; and, whatever he may pretend, is in heathen ignorance, or even worse than heathen ignorance, to the present time, notwithstanding the clear shining of the light of the Gospel.