Waffle.

Religion is a schematic subset of spirituality.

Spirituality is an emotional reaction to ignorance, awe and familiarity.

Religion is when humans take this spiritual wonder and use it to achieve dominance over others. The priesthood in Ancient Mesopotamia will have held "Sacred Knowledge" including the calendar. The calendar was important to agriculture obviously, and agriculture was the basis of the food surplus which allowed the cultural centres and later the great cities.

The priesthood also controlled the cities, including Babilu (or Babylon or Babel)where Marduk ('The Solar Calf') was the patron deity and where one of the earliest known codes of law, Hammurabi's Code, was written which separated the powers of the gods (as performed by the priesthood) and man (as performed by the King).

The fates of the gods was tied into the fates of man and as cities grew more powerful then their gods would take on new aspects gained in the heavenly battle. The priesthood probably convinced the people that their god was really fighting and they fought merely to inspire him (or her - the Mesopotamians weren't sexist) to their victory and that if they lost it was their fault for failing to inspire their patron to victory. Or something equally daft.

Anyway, this practice continued (OK, maybe not in the exact form I described) until the followers of some bloke called Jahweh decided their god had won and killed anyone who disagreed.