A few texts such as "Atharvashiras Upanishad" mention Rudra, and assert all gods are Rudra, everyone and everything is Rudra, and Rudra is the principle found in all things, their highest goal, the innermost essence of all reality that is visible or invisible. The "Kaivalya Upanishad" similarly, states Paul Deussen – a German Indologist and professor of Philosophy, describes the self-realized man as who "feels himself only as the one divine essence that lives in all", who feels identity of his and everyone's consciousness with Shiva (highest Atman), who has found this highest Atman within, in the depths of his heart.
The Shaiva Puranas, particularly the Shiva Purana and the Linga Purana, present the various aspects of Shiva, mythologies, cosmology and pilgrimage ("Tirtha") associated with him. The Shiva-related Tantra literature, composed between the 8th and 11th centuries, are regarded in devotional dualistic Shaivism as Sruti. Dualistic Shaiva Agamas which consider soul within each living being and Shiva as two separate realities (dualism, "dvaita"), are the foundational texts for Shaiva Siddhanta. Other Shaiva Agamas teach that these are one reality (monism, "advaita"), and that Shiva is the soul, the perfection and truth within each living being. In Shiva related sub-traditions, there are ten dualistic Agama texts, eighteen qualified monism-cum-dualism Agama texts and sixty-four monism Agama texts.
Shiva-related literature developed extensively across India in the 1st millennium CE and through the 13th century, particularly in Kashmir and Tamil Shaiva traditions. The monist Shiva literature posit absolute oneness, that is Shiva is within every man and woman, Shiva is within every living being, Shiva is present everywhere in the world including all non-living being, and there is no spiritual difference between life, matter, man and Shiva. The various dualistic and monist Shiva-related ideas were welcomed in medieval southeast Asia, inspiring numerous Shiva-related temples, artwork and texts in Indonesia, Myanmar, Cambodia, Laos, Vietnam, Thailand and Malaysia, with syncretic integration of local pre-existing theologies.
See also: Roots of Hinduism
The figure of Shiva as we know him today may be an amalgamation of various older deities into a single figure. How the persona of Shiva converged as a composite deity is not understood, a challenge to trace and has attracted much speculation. According to Vijay Nath, for example:Vishnu and Siva [...] began to absorb countless local cults and deities within their folds. The latter were either taken to represent the multiple facets of the same god or else were supposed to denote different forms and appellations by which the god came to be known and worshipped. [...] Siva became identified with countless local cults by the sheer suffixing of "Isa" or "Isvara" to the name of the local deity, e.g., Bhutesvara, Hatakesvara, Chandesvara."An example of assimilation took place in Maharashtra, where a regional deity named Khandoba is a patron deity of farming and herding castes. The foremost center of worship of Khandoba in Maharashtra is in Jejuri. Khandoba has been assimilated as a form of Shiva himself, in which case he is worshipped in the form of a lingam. Khandoba's varied associations also include an identification with Surya and Karttikeya.
Lingodbhava is a Shaiva sectarian icon where Shiva is depicted rising from the Lingam (an infinite fiery pillar) that narrates how Shiva is the foremost of the Trimurti; Brahma and Vishnu are depicted bowing to Lingodbhava Shiva in the centre.
Main articles: Shaivism and History of Shaivism
Shaivism is one of the four major sects of Hinduism, the others being Vaishnavism, Shaktism and the Smarta Tradition. Followers of Shaivism, called "Shaivas", revere Shiva as the Supreme Being. Shaivas believe that Shiva is All and in all, the creator, preserver, destroyer, revealer and concealer of all that is. He is not only the creator in Shaivism, but he is also the creation that results from him, he is everything and everywhere. Shiva is the primal soul, the pure consciousness and Absolute Reality in the Shaiva traditions.
The Shaivism theology is broadly grouped into two: the popular theology influenced by Shiva-Rudra in the Vedas, Epics and the Puranas; and the esoteric theology influenced by the Shiva and Shakti-related Tantra texts. The Vedic-Brahmanic Shiva theology includes both monist ("Advaita") and devotional traditions ("Dvaita") such as Tamil Shaiva Siddhanta and Lingayatism with temples featuring items such as linga, Shiva-Parvati iconography, bull Nandi within the premises, relief artwork showing mythologies and aspects of Shiva.
The Tantric Shiva tradition ignored the mythologies and Puranas related to Shiva, and depending on the sub-school developed a variety of practices. For example, historical records suggest the tantric Kapalikas (literally, the 'skull-men') co-existed with and shared many Vajrayana Buddhist rituals, engaged in esoteric practices that revered Shiva and Shakti wearing skulls, begged with empty skulls, used meat, alcohol, and sexuality as a part of ritual. In contrast, the esoteric tradition within Kashmir Shaivism has featured the "Krama" and "Trika" sub-traditions. The Krama sub-tradition focussed on esoteric rituals around Shiva-Kali pair. The Trika sub-tradition developed a theology of triads involving Shiva, combined it with an ascetic lifestyle focusing on personal Shiva in the pursuit of monistic self liberation.
The Vaishnava (Vishnu-oriented) literature acknowledges and discusses Shiva. Like Shaiva literature that presents Shiva as supreme, the Vaishnava literature presents Vishnu as supreme. However, both traditions are pluralistic and revere both Shiva and Vishnu (along with Devi), their texts do not show exclusivism, and Vaishnava texts such as the "Bhagavata Purana" while praising Krishna as the Ultimate Reality, also present Shiva and Shakti as a personalized form an equivalent to the same Ultimate Reality. The texts of Shaivism tradition similarly praise Vishnu. The Skanda Purana, for example, states:Vishnu is no one but Shiva, and he who is called Shiva is but identical with Vishnu.
— Mythologies of both traditions include legends about who is superior, about Shiva paying homage to Vishnu, and Vishnu paying homage to Shiva. However, in texts and artwork of either tradition, the mutual salutes are symbolism for complementarity. The Mahabharata declares the unchanging Ultimate Reality (Brahman) to be identical to Shiva and to Vishnu, that Vishnu is the highest manifestation of Shiva, and Shiva is the highest manifestation of Vishnu.
Ardhanarishvara sculpture, Khajuraho, depicting Shiva with goddess Parvati as his equal half. In the Ardhanarisvara concept, the icon is presented as half-man and half woman.
The goddess-oriented Shakti tradition of Hinduism is based on the premise that the Supreme Principle and the Ultimate Reality called Brahman is female (Devi), but it treats the male as her equal and complementary partner. This partner is Shiva.
The earliest evidence of the tradition of reverence for the feminine with Rudra-Shiva context, is found in the Hindu scripture "Rigveda", in a hymn called the Devi Sukta:3. I am ruler, assembler of goods, observer foremost among those deserving the sacrifice.
Me have the gods distributed in many places—so that I have many stations and cause many things to enter (me).
4. Through me he eats food—whoever sees, whoever breathes, whoever hears what is spoken.
Without thinking about it, they live on me. Listen, o you who are listened to: it’s a trustworthy thing I tell you.
5. Just I myself say this, savored by gods and men:
"Whom I love, just him I make formidable, him a formulator, him a seer,
him of good wisdom".
6. I stretch the bow for Rudra, for his arrow to smash the hater of the
sacred formulation.
I make combat for the people. I have entered Heaven and Earth.