To The Skeptic's Dictionary - Table of Contents
An oracle is a shrine or temple sanctuary consecrated to the worship and consultation of a prophetic god. The person who transmits prophecies from a deity at such a shrine is also called an oracle (or prophet), as is the prophecy itself. The prophecy made known at such a shrine is often in the form of an enigmatic statement, or allegory. In the Judeo-Christian traditions, oracles are called prophecies or revelations.
Even if we grant that there is a God who only speaks to special people, how would one ever know the difference between a delusion and the real thing? How would anyone, who did not have a revelation, know that a person who claims to have had one really had one?
Oracles and revelations are the words of human beings. Only faith gives the words divine status.
Ask yourself what kind of person would be willing to kill his only son (or any other human being, for that matter) because in a vision or dream, an angel told him it was God's will. What kind of person would not immediately identify such a dream or vision as a delusion or nightmare? People are always looking for excuses to justify their evil behavior, and the will of God is the greatest excuse of them all. People are always looking for ways to manipulate other people, too, and the power of God is as good as a gun to the head of a willing disciple.
Some alleged prophets are not associated with any religion, e.g. Nostradamus.
I suppose the belief in oracles can be traced to the desire to know the future. Unfortunately, the only sure guide to the future is the past, and even that isn't always reliable. Because some prophecies are more or less accurate, however, believers think they have good evidence for the reality of prophecy. Yet, many prophecies are ambiguous and open to several interpretations, making it possible to fit a wide array of events, some contradictory, into the predictive circle. Some prophecies are made after the fact. For example, after Hitler was dead and gone, it was discovered that Nostradamus had predicted the whole Nazi affair, centuries ago. Some prophecies are correct due to chance. Some chance events seem highly improbable to people when in fact they are not that improbable at all. Predicting that there will be an earthquake in California is no great shakes since there are several thousand every year. Predicting an American soldier will be killed by a land mine in Bosnia is not as risky a prediction as it might sound. Finally, belief in oracles is aided by selective thinking. We remember the hits and forget the misses.
See related entries on God and the I Ching.