Oldest of the world's major religions. It evolved from the Vedic religion of ancient India. The major branches of Hinduism are Vaishnavism and Shaivism, each of which includes many different sects. Though the various sects each rely on their own set of scriptures, they all revere the ancient Vedas, which were likely composed about the mid-2nd millennium BCE. The philosophical Vedic texts called the Upanishads explored the search for knowledge that would allow humankind to escape the cycle of reincarnation. Fundamental to Hinduism is the belief in a cosmic principle of ultimate reality called brahman and its identity with the individual soul, or atman. All creatures go through a cycle of rebirth, or samsara, which can be broken only by spiritual self-realization, after which liberation, or moksha, is attained. The principle of karma determines a being's status within the cycle of rebirth. The Hindu deities having the widest following are Vishnu and Shiva, who are worshipped in various avatars, or incarnations. The goddess Durga also has a wide following. The major sources of classical stories about the gods are the Mahabharata (which includes the Bhagavadgita, one of the most important religious texts of Hinduism), the Ramayana, and the Puranas. Historically, the hierarchical social structure of the caste system was also important in Hinduism. In the 20th century Hinduism blended with Indian nationalism to become a powerful political force in Indian politics. In the early 21st century there were more than 850 million Hindus worldwide.