Revelation 7:14
Revelation 7:1414“Sir,” I answered, “you know.” So he replied, “These are the ones who have come out of the great tribulation; they have washed their robes and made them white in the blood of the Lamb.
Watchtower commentary takes as foundational to the great-tribulation doctrine. The "great crowd" who "come out of the great tribulation" are the earthly-hope class who survive Armageddon — having washed their robes in the Lamb's blood by associating with Jehovah's organisation. The tribulation is a future event in which the United Nations destroys all false religion, then turns on Jehovah's Witnesses before Jehovah brings paradise earth at Armageddon.
Read alongside . Where Matthew says "the abomination of desolation" (which the Watchtower takes as a future UN attack), Luke records the same teaching with the explicit explanation "Jerusalem surrounded by armies." The great tribulation is the AD 70 destruction of Jerusalem, not a future JW-specific persecution. The "great crowd" washing their robes in the Lamb's blood are the redeemed of every age — drawn from "every nation, tribe, people, and tongue" (v9) — not a separate earthly class. They stand before the throne (heaven, per 4:2 and 5:11), serving God day and night in his temple (naos — heavenly throughout Revelation). The Watchtower bunker-doctrine of a future tribulation targeting JWs has no textual anchor outside Watchtower commentary.