The Origin of the Four Castes According to Manu

Manu is considered the lawgiver of Aryan Brahmanism. In fact, the very first chapter, which is a creation account, of the Manusmriti pronounces the creation and division of castes (varna. The castes are said to proceed from the very body of God.

for the sake of the prosperity of the worlds he caused the Brahmana, the Kshatriya, the Vaisya, and the Sudra to proceed from his mouth, his arms, his thighs, and his feet. (1:31)

The duties (Varna dharma) of the four castes were prescribed by Manu with a theological rationale:

87. But in order to protect this universe He, the most resplendent one, assigned separate (duties and) occupations to those who sprang from his mouth, arms, thighs, and feet.
88. To Brahmanas he assigned teaching and studying (the Veda), sacrificing for their own benefit and for others, giving and accepting (of alms).
89. The Kshatriya he commanded to protect the people, to bestow gifts, to offer sacrifices, to study (the Veda), and to abstain from attaching himself to sensual pleasures;
90. The Vaisya to tend cattle, to bestow gifts, to offer sacrifices, to study (the Veda), to trade, to lend money, and to cultivate land.
91. One occupation only the lord prescribed to the Sudra, to serve meekly even these (other) three castes.

(The Laws of Manu, George Bühler, translator [1886] (Sacred Books of the East, vol. 25))

Manu's laws strongly favor and are oriented towards the caste social system.

The system was largely exploited by the upper castes to oppress the Sudras for centuries until "untouchability" was abolished by the Indian Constitution in 1950. However, caste-consciousness as a social-status has not fully disappeared. Sadly, especially in the South of India, even Christians are not liberated of caste-observance yet. Though theologically denounced, a large number of them still consider themselves to be inextricably bound to it (See Encyclopedia Britannica Article, Christian Caste).