Fortune-telling was a central part of many ancient classical religions, knowledge of the future, or a least a belief in having knowledge of the future, providing some bulwark against the fragility of life. Whole cities and states officially consulted the great oracles such as the Pythia at Delphi. The fabulously wealthy King Croesus of Lydia had many centuries before, according to Herodotus, asked the Pythia whether he should attack Persia. He had received a typically ambiguous response: “If you do, you will destroy a great empire.” Heartened by this, he immediately attacked, only to discover that the empire the Pythia was referring to was his own.
Even Alexander had sought the oracle at Siwa to discover if his campaigns would be successful. This may seem like a piece of stage magic today, but the word of the Siwa oracle not only helped Alexander to resolve his course of action, but likely paved the way for his conquests. In the same way, not long after the Chaldean oracles of Babylon began predicting his doom, he did indeed die, and the same doleful prophesying had probably helped to oust Darius before that. Prophecies could be self-fulfilling. Enemies would attack a man marked out for bad luck. A man apparently blessed would be left alone. Oracles, to put it simply, worked, and many of all types and importance vied with each other. At the greatest, such as at Delphi and Siwa, kings themselves might send for answers, receiving back those cryptic messages from the god via a human intermediary, usually a priest or priestess absorbed in an ecstatic trance or under the influence of psychotropic drugs. Romans would seek answers in the entrails of sacrificed animals, as interpreted by their augurs.
— The Rise and Fall of Alexandria, pp. 182-183