Jamieson, Fausset and Brown on Isaiah 8:20

20. To the law, &c.--the revelation of God by His prophet (Is 8:16), to which he directs them to refer those who would advise necromancy.

if they speak not ... it is because--English Version understands "they" as the necromancers. But the Hebrew rendered "because" is not this but "who"; and "if not," ought rather to be "shall they not"; or, truly they shall speak according to this word, who have no morning light (so the Hebrew, that is, prosperity after the night of sorrows) dawning on them [Maurer and G. V. Smith]. They who are in the dark night of trial, without a dawn of hope, shall surely say so, Do not seek, as we did, to necromancy, but to the law," &c. The law perhaps includes here the law of Moses, which was the "Magna Charta" on which prophetism commented [Kitto].