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. If ye be willing and obedient, ye shall eat the good of the land: but if ye refuse and rebel, ye shall be devoured with the sword; for the mouth of Jehovah hath spoken it."

The proper understanding of this passage as a reference to the new covenant which was destined eventually to replace the Law of Moses is faithfully witnessed by the unerring instinct of the Church which has incorporated these words into their hymnals all over the world. Jeremiah indicated that the forgiveness of sins was an infallible identifying mark of the New Covenant (Isaiah 31:35). Thus we have further proof that Isaiah in this chapter is not merely describing the results of some undetermined invasion; but he is speaking of the New Covenant, the "faithful remnant" and the Church of Jesus Christ.

Also, we should notice that obedience, faithful and loyal obedience, is the sine qua non with regard to that redemption which includes forgiveness. The threat of the sword for the disobedient is a statement negatively of the same universal and eternal principle.