Charles Ellicott Commentary Isaiah 1:18

"Come now, and let us reason together, saith Jehovah: though your sins be as scarlet, they shall be as white as snow; though they be red like crimson, they shall be as wool." — Isaiah 1:18 (ASV)

Come now, and let us reason together. — The Authorized Version suggests the idea of a discussion between equals. The Hebrew implies rather the tone of one who gives an authoritative ultimatum, as from a judge to the accused, who had no defense, or only a sham defense, to offer (Micah 6:2–3). “Let us sum up the pleadings—that ultimatum is one of grace and mercy—‘Repent, and be forgiven.’”

Though your sins be as scarlet. — The two colors probably corresponded to those now designated by the English words. Both words point to the dyes of Tyre, and the words probably received a fresh emphasis from the fact that robes of these colors were worn by the princes to whom Isaiah preached (2 Samuel 1:24). To the prophet’s eye, that dark crimson was like the stain of blood.

What Jehovah promises is that the guilt of the past, however deep-dyed in grain it might be, should be discharged, and leave the character with a restored purity. Men might dye their souls of this or that hue, but to bleach them was the work of God. He alone could transfigure them so that they would be white as snow (Mark 9:3). Compare the reproduction of the thought, with the added paradox that it was the crimson blood of the lamb that was to bleach and cleanse, in Revelation 3:4-5; Revelation 7:14.