I mean, if you are an illiterate tribesman, and you see some guy making sense out of what looks to you like a bunch of jumbled markings, you might try to one up him in order to look important to your other illiterate superstitious buddies.

Ancient man was trying to make sense out of the natural world. Maybe "rolling the bones" started as some child's game, but suddenly a child could predict outcomes with them. Maybe an unusual intestine was found in a butchered animal at an "auspicious" event.

Seeing the future into sacred wells, rivers, and lakes also have a plausible explanation, especially if any drugs were involved.

They were actually the philosophers and scientists of their times, learning and trying to make sense of the world and the environment in which they lived.

Someone once proposed that astrology got started, when ancient men recognized certain star patterns coordinated with the seasons, and could tell you when the best time to plant your seeds, or to predict the migratory patterns of animals. Basically, they were using the stars as a calender, and there were certain people who could recognize them better then others who became priest.

Over time, the priest tried to grab more power, so they tried to apply the stars to other aspects of life. More times then not, they were wrong, but random chance along with hedged bets allowed them to guess right just enough that it appeared magical, and thus religion was born.

The combination of markings to form words and specific language rules for something like spelling is actually a part of the visual parts of our brain. Most humans, illiterate or not, recognize the difference between a jumble of symbols and actual letters that follow a language pattern. In fact, a really cool study was just recently published which showed that one year old babies and monkeys have the same ability (which is pretty rad, if I do say so myself).

My thoughts are that they were doing what pretty much every church leader has done for centuries upon centuries. They are using superstition and fear to gain a high ranking spot in their own communities.