Religion

Some people believe that we have guardian angels who fly around on magic carpets wearing bomber jackets. Other people believe that when we die, we go to a gay club in Charing Cross called Heaven. There are others who assert that if you pray loudly enough, you'll be able to hear your own voice echoing back from God's ears.
All these are forms of religious belief.

Religious belief has been a part of human culture and society ever since we invented guilt. Since then (humans being fractious creatures by nature) we have subdivided belief into separate religions such as Christianity, Islam, Buddhism, and Moomin. Each of these is subdivided into multiple denominations: Christians, for example, can be Roman Catholic, Anglican, Methodist or Sensible Jumper. Each denomination is further split into factions, and so on down the line until every individual essentially has his or her very own religion.

This is ideal, because it means you can believe what you want, and anyone who disagrees with you is at best misguided and at worst eternally damned, so you can kill them and take their stuff.