Deuteronomy 18:22
Deuteronomy 18:2222When a prophet speaks in the name of the LORD and the message does not come to pass or come true, that is a message the LORD has not spoken. The prophet has spoken presumptuously. Do not be afraid of him.
JWs accept as God's standard for testing prophets but insulate the Watchtower from the test by insisting they "never claimed to be inspired prophets — only imperfect men doing their best." Failed predictions about 1874, 1914, 1925, 1975 and the literal-then-overlapping "generation" of 1914 are recategorised as "expectations" or "imperfect human understanding" rather than as messages spoken in Jehovah's name. ("the light gets brighter") is then deployed to explain away each reversal.
Deut 18:22 says "when a prophet speaks in the name of Jehovah and the word does not occur or come true, that is a word that Jehovah did not speak." The standard is perfection, the test is the outcome, and the insulation language ("we never said inspired") does not survive contact with the Watchtower's own published claims. The 1922 Watchtower described its dates as carrying "the stamp of approval of Almighty God." A 1968 Watchtower called the 1975 chronology "absolutely and unqualifiedly correct." The Awake! masthead promised for decades to publish only what was "of divine origin." If predictions made under those banners fail, Deut 18:22 applies — by the Watchtower's own framing, not by an outsider's. cannot rescue this: brightening light adds detail; it does not reverse a specific failed prediction. Coupled with Deut 13 (a prophet who diverts you to a different god is to be rejected even if the signs come true), the verse is the master test the Watchtower must face on its own published record.