Albert Barnes Commentary Isaiah 8:20

"To the law and to the testimony! if they speak not according to this word, surely there is no morning for them." — Isaiah 8:20 (ASV)

To the law ... - To the revelation which God has given. This is a solemn call of the prophet to test everything by the revealed will of God; see Isaiah 8:16.

If they speak not - This refers to the necromancers—those who pretended to have contact with the dead.

According to this word - That is, according to what God has revealed. By this standard all their pretended revelations were to be tested. By this standard all doctrines are still to be tested.

It is because - There has been a great variety of criticism on this verse, but our translation probably expresses the true idea. The word rendered here ‘because,’ אשׁר 'ăsher—which commonly denotes ‘which’—seems here to be used in the sense of the Syriac “Dolath,” or the Greek ὅτι hoti.

No light - The margin reads, ‘Morning.’ The Hebrew word is שׁחר shāchar. This word usually means the morning light: the mingled light and darkness of the aurora, or daybreak. It serves as an emblem of advancing knowledge and perhaps also of prosperity or happiness after calamity, just as the break of day succeeds the dark night.

The meaning here may be: ‘If their teachings do not accord with the law and the testimony, it is proof that they are totally ignorant, without even the twilight of true knowledge; that it is total darkness with them.’

Alternatively, it may mean, as Gesenius suggests: ‘If they do not speak according to this word, then no dawn will arise; that is, no prosperity will smile upon this people.’

Lowth understands it as referring to obscurity or darkness:

‘If they speak not according to this word,
In which there is no obscurity.’

However, there is no evidence that the word shāchar is ever used in this sense of obscurity. Others suppose that the Arabic sense of the word—deception or magic—should be retained here: ‘If they speak not according to this oracle, in which there is no deception.’ But the word is not used in this sense in Hebrew.

The meaning is probably this: The law of God is the standard by which all professed communications from the invisible world are to be tested. If the necromancers deliver a doctrine that is not sustained by that law and is not in accordance with prophetic communications, it shows that they are in utter ignorance. For them, there is not even the glimmering of morning twilight; all is total night, error, and obscurity, and they are not to be followed.