Bridges on Proverbs 20:20
Charles Bridges on Proverbs 20:20

20.  Whoso curseth his father or his mother, his lamp shall be put out in obscure darkness. {lamp: or, candle}
If darkness be the punishment, is it not also the cause of this atrocious sin? For surely even the light of nature must be extinguished, ere the child should curse even those who under God have taught it to speak — the authors and preservers of its existence; its greatest earthly benefactors. Even an undutiful look, much more a word, is an offense against the commandment. What then must be the weight of guilt involved in the cursing of them! The deepest reverence is due to them when they are dead. (Jeremiah 35:1-10.) What then must be the provocation of sinning against them while they are living for their children, in all the active, self-denying energy of love and service! This cursing, according to our LORD’s standard, includes “setting light by father or mother;”† willful disobedience — a fearful, palpable mark of the last days.† How God regards it, let his own curse on Mount Ebal (Deuteronomy 27:16), and his judgment of temporal death,† testify. The present degradation of Africa is a witness, on the confirming page of history, of the frown upon an undutiful son (Genesis 9:22-25) — his lamp put out in darkness.† And if the temporal sentence of death is repealed, the more awful judgment still remains unchangeably upon the Divine statute-book — obscure darkness — “the blackness of darkness” — darkness eternal without a ray of light, of which “blackness” is only the shadow, to shew what the substance must be.