[RobbyS]

Hinduism is a synthesis of the polytheism of one group and the "monotheism" of people from southwest Asia. so I gather. That is one reason for the development of the caste system.

[CarrotAndStick]

More than a synthesis, it is actually not a religion in a strict sense, rather a collection of philosophies and ideas. Hence its extremely large variety in form, and also its self-contradictory nature (this isn't a feature of Hinduism alone, but of most other faiths). And the brunt of these ideas pre-date the South-West Asian monotheism.