The Mekhilta, the halakhic midrash on Exodus from the 2nd century CE, examines one of the starkest either-or passages in the Prophets. Isaiah delivers God's ultimatum: "If you acquiesce and pay heed, the good of the earth will you eat. But if you refuse and rebel, the sword will devour you; for the mouth of the Lord has spoken" (Isaiah 1:19-20). Obey and prosper, or rebel and die by the sword. The prophet then seals it with the phrase: "the mouth of the Lord has spoken."
The rabbis ask their characteristic question — where did God first speak this? The Mekhilta points to (Leviticus 26:25): "I will bring against you an avenging sword." This verse appears in the Tokhechah, the terrifying section of curses that God promises will fall upon Israel if they violate the covenant. The sword Isaiah warns about is the very same sword God threatened centuries earlier at Sinai.
Isaiah is not innovating. He is reminding. The prophet is pointing the people of Judah back to the original terms of the covenant — terms that were established before they ever entered the land. The blessings and curses were set in stone at Sinai, and every later prophet who invokes them is simply calling Israel back to what they already agreed to.
The Mekhilta's method demonstrates that prophecy does not introduce new threats or promises. It reactivates old ones.