Isaiah 8:20

To the law and to the testimony: if they speak not according to this word, it is because there is no light in them.

To the law and to the testimony - To the revelation of God by His prophet (Isaiah 8:16), he directs them to refer those who would advise necromancy.

If they speak not according to this word, (it is) because (there is) as light in them. The English version understands "they" as the necromancers. But the Hebrew ( 'ªsher (H834)) rendered because, is who: and the Hebrew for if not may be, 'shall they not,' or, 'truly they shall speak according to this word, namely, everyone upon whom (the Hebrew relative is singular) no morning light is dawning' - i:e., who have no morning light (so the Hebrew [shaachaar] - i:e., no prosperity after the night of sorrows) dawning on (i:e., awaiting) them' (Maurer). Too late they shall see their deadly error. The law includes here the law of Moses, the 'Magna Charta' on which all prophecy rests: also all Scripture. Calvin and Grotius support the English version. 'If they turn from the word of God as the only infallible oracle, it indicates that, or it is because (so the Hebrew relative, asher, means, because in 1 Kings 15:13 and Psalms 119:158) there is no light of truth in any one of them; or, as Grotius explains the latter clause, 'it indicates that no light of prosperity shall attend any one of them.' This latter accords best with the description of the adversity which follows under the image of darkness and anguish about to be the portion of those who have deserted the Word of God for lying spirits. So "light" is used for prosperity and joy (Esther 8:16; Psalms 97:11). No light of relief shall succeed to the perpetual night of their calamity, as dawn succeeds night (Job 11:17).