You may say to yourselves, "How can we know when a message has not been spoken by the LORD?"
If what a prophet proclaims in the name of the LORD does not take place or come true, that is a message the LORD has not spoken. That prophet has spoken presumptuously, so do not be alarmed.
Only predictions that come true are from God, you see, while failed prophecies are just down to human slip-ups—a truly divine response to anyone who would condemn either a prophet or a whole belief system on the minor matter of a failed apocalypse.
Even without this sacred disclaimer, it's easy enough for a believer to reinterpret and revise the details of a prediction so that it fits whatever facts are on the ground. The research literature is littered with such examples. When atomic energy didn't sweep over the Earth to herald the Second Coming on Christmas Day 1967, the Universal Link group cheerfully reinterpreted their prophecy as pertaining to a spiritual force, rather than a physical effect. When flying saucers never announced their presence to humankind in 1976, the Unarian sect gently reworked its prophecy to refer more broadly to some point in "the future," while blaming limited human minds for misunderstanding the aliens' grand plan. When a Pentecostal group led by the God-channelling housewife Mrs. Shepard emerged after more than a month from self-built fallout shelters, they were pleased that the divinely ordained nuclear holocaust had not come to pass—and grateful for having passed a test of their faith.
In fact, so many studies have been conducted on unfulfilled prophecies from religions large and small that they were compiled into a fascinating book from 2000, Expecting Armageddon. None of the groups described reacted to the unexpected persistence of the world with a zealous drive for new members, and most made just minor adjustments to their beliefs. If Harold Camping's followers remain steadfast in their devotion come Sunday afternoon, don't be surprised—it's merely a testament to the human spirit.
For those not waiting for the world to end in a storm of fire and light it is easy to write off the believers as deluded, but Festinger was not so wide of the mark when he suggested that we adapt to even the most unlikely of contradictions using nothing more than our methods of everyday rationalization. The faithful could just as easily be those who stubbornly stand by disgraced politicians, failed ideologies, dishonest friends, or cheating spouses, even when reality highlights the clearest of inconsistencies. Armageddon is unlikely to arrive this weekend, but most of us have lived through it many times before.