Perhaps an independent oracle, or even two (Isaiah 1:18 and Isaiah 1:19 f.); the date is quite uncertain. According to the usual view Yahweh challenges Israel to a lawsuit, that His righteousness may be vindicated and its guilt clearly seen. But it is not certain that a legal process is implied. Nor is Isaiah 1:18 clear. It may be a gracious invitation (so RV), it may be sarcastic (let them be white as snow!), or an indignant question. The last is grammatically uncertain, but it gives the best sense: If your sins are as scarlet, how should they be reckoned white as snow? if they are red like crimson, how should they be as wool? No distinction is intended between scarlet and crimson.
Isaiah 1:19 f. is a characteristic expression of the earlier view that righteousness and prosperity were inseparably associated.
Isaiah 1:20. devoured with the sword: better, ye shall eat the sword, an effective contrast to Isaiah 1:19; but Cheyne's emendation, on husks (harubim) shall ye feed, is tempting. The husks are the carob-pods on which the Prodigal fed the swine (Luke 15:16).